Web Design in a Nutshell, 3rd Edition

Adjacent GIFs and JPEGs

There are times when you want abutting GIF and JPEG graphics to blend seamlessly together, as in the case of an image that has been sliced up and held together with a table. Unfortunately, there is no way to preserve specific numerical RGB settings in a JPEG because of its lossy compression scheme. So although the images may start out with the exact same RGB color in the abutting edges, in the JPEG, that color has the potential to get blotchy and shift, while in the GIF, it is preserved in an index color table. The severity of this effect is dependent on the type of image and the degree of JPEG compression.

The only true solution is to make all adjacent graphics GIFs or 8-bit PNGs so the RGB values can be maintained. Otherwise, play around with higher quality JPEG compressions to minimize the difference.