loss less image compression

Every time you save a jpeg, you have some loss.. Nature of the file format.
A better way, sorry that I don't know..

Please be clear: are you actually looking to reduce the number of pixels in the image (say, from 3000 pixels wide to 600 pixels wide), or are you just looking to change the DPI meta data in the files? These are not the same thing.

"Upload to Web" and "Lossless Compression" don't really go hand in hand. Just what are you talking about?

Every time you save a jpeg, you have some loss.. Nature of the file format.

sorry but that is a myth !

A number of alterations to a JPEG image can be performed losslessly as long as the image size is a multiple 1 MCU block (Minimum Coded Unit) (usually 16 pixels in both directions, for 4:2:0 chroma subsampling ). Utilities that implement this include jpegtran , with user interfaceJpegcrop, and the JPG_TRANSFORM plugin to IrfanView .

further there are very simple algorithms just flipping the matrix order or spoken naturally the "orientation" of native blocks.

I'm sure you know better, but just in case, do not downsize the original files. Use "save as" to save lower-res copies either under a slightly different name or in a different place.

And to add a point, most monitors these days resolve at 96 pixels per inch, so for the web, you might consider downsizing to 96 ppi. rather than 72 ppi.

Peter

One reason I could not function without Lightroom. We can keep the original files as is, and do all conversions so easy, including importing to PS for things LR does not do. Using a plugin, can size files even as they are exported to my website. Then go do something else as it does. If I want tiff's, it converts and sizes them. If I want JPGs can set the pixel dimensions and resolution to just what I want and just archive the originals.
If you have to do it frequently think it would save lots of time.

In this context, the ppi resolution is meaningless. For web images, it doesn't matter what resolution you set, because that applies only to print.
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For the web, you need to size the images in absolute pixels dimensions -- for example 800 px on the long side, or 1024 on the long side, something like that. When you save the tiffs as jpegs, the compression isn't lossless. But it also shouldn't harm image quality if you use the right amount of compression. In Photoshop, I would use quality level 7 to 10, depending on how much I wanted to limit file size. If you don't have Photoshop, you can use IrfanView (free) to resize the images and save as jpegs. IrfanView can also do them in a batch.

Just to illustrate that resolution (ppi) has no meaning on the web, these were saved at the resolutions shown:

The reason I mentioned 96 ppi is because when I am preparing my images for the web, I generally think in inches rather than pixels. I can't help it, inches have been ingrained in my brain when thinking about image size. I think to myself, "this image will look good if I can display it on the web at about 5"x7" (for instance)" In that case I need to know that monitors resolve at 96 ppi, so I can size my image at 5"x7" @ 96ppi.

The original poster had the same thoughts I think, because of the way the question was posed. "I now need to upload these 50 jpeg images on the web and I'd like to reduce the resolution to 72 ppi." I took that to mean that the length and width were to remain the same, but resolution was to be changed to display on the web. In that context, 96 ppi is the better figure to downsize to.

Peter

I took that to mean that the length and width were to remain the same, but resolution was to be changed to display on the web. In that context, 96 ppi is the better figure to downsize to.

It may how you prefer to think about image size, but I'm not sure it's a better way, for this reason: It doesn't take into account the various screen resolutions that different people use. For web pages designed to display images, there's an advantage to thinking in terms of pixels and sizzing images with a specific screen resolution in mind.
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For example, suppose you have a landscape pano that you want to display as large as possible. If you aim for a particular screen resolution (such as 1024 x 768), you can size the image to fill that screen without being resized. Obviously, you can't optimize the image for other resolutions, but you can at least hit a target that is statistically (for your site) the most prevalent. To do that, the most direct route is to size the image in terms of pixel dimensions, not print dimensions. That's why web designers pay so much attention to screen resolution statistics.

Please, please, listen to David. So many people get so tangled up about resolution and pixels per inch for Web stuff, when it is only about the number of pixels in the image. And much easier to understand than ppi. Remember, ppi is for print.

ppi is for print.

Just to explain this - in "ppi", the "i" is for "inches." Screens don't display in inches, they display in pixels. Ignore the ppi (or dpi in most applications) until you print. Prints have inches, screens don't.

Explain the correlation between screen resolution to image resolution in this screenshot of an image destined for the web zoomed in at maximum 1600% in CS3. The actual data is accurate as can be seen by counting the pixels per 1/4 section of Photoshop's ruler guide. Image Size is in agreement.

However, there's a disconnect between the actual size of an inch in the cyber world of Photoshop compared to screen resolution of a display. They don't match up.

1680 wide display which measures at 16.80 inches wide measured with an actual ruler on my iMac. 1680 divided by 16.80 inches=100 (pixels?) per inch.

100 x .75-(the .75 derived from 3/4 inch actual size of one inch measuring from CS3's rulers at 100% view measured with an actual ruler)=75 ppi, close to 72 which is the Mac's subgraphic pixel grid used in its page buffer on the video card. Windows I believe uses a 96 pixel subgraphic pixel grid.

Correct me if I'm wrong on this especially on the math. Not sure about that 100 x .75 equation. Also not sure if Mac's still use the 72 subgraphic pixel grid layout.

Thanks for the advice everybody. Puts things in place for me.
David Thomasson: Thanks for your input. You clarified the doubts I had.

Sowmya