I don't understand how dpi cannot be relevant for images on the web. I've attached a number of images - these were from hi res tiffs - the first is an image 100mm wide @ 300dpi and the second 100mm wide @100dpi - both resized in PS and saved as quality 10 jpgs. The third image - a screen capture, shows both images at 100% - obviously the one is larger, but the quality of the images appears to be the same. The last screen capture shows both images at the same size on screen with a vast difference in quality. So if you want high quality images on your website viewed at a certain size, the dpi of the images is very important.

Click image for larger version.  Name: 300dpi 100mm.jpg  Views: 128  Size: 72.1 KB  ID: 106334Click image for larger version.  Name: 100dpi 100mm.jpg  Views: 139  Size: 70.3 KB  ID: 106333Click image for larger version.  Name: 100%.jpg  Views: 132  Size: 62.4 KB  ID: 106332Click image for larger version.  Name: same size.jpg  Views: 132  Size: 88.2 KB  ID: 106335