Looks like most of the screens are 200KB and up. Mine is 900KB as .PNG, resaved as .JPEG yields 127KB. Simplicity tweak went from 6KB .PNG to 20KB .JPEG. DSTmod went from 1.8MB (!) .PNG to 229KB .JPEG. Glazemod went from 623KB .PNG to 138KB. Statistically, it looks more like the images are compressing a lot better in .JPEG than .PNG. It may be possible, however, that people are uploading uncompressed .JPEG files and so a reduction in size via .PNG is possible. However, an optimised .JPEG compression through ACDSee 5.0, for instance, yields a highly compressed image that is much smaller than a .PNG file.

-Jon