General Images

Allow auto zoom out/full view of pictures when viewing images in Gmail
Allow custom zoom levels
Ability to change the default background color/image (useful for images with alpha values).
Preload longdesc page and display as popup, below image or with visibility hidden (user settable)
Ability to choose whether to show broken images or not. Seriously, it can be a pain for developers if they can't see at least a crude red 'X' where the image should be. of course, just a red X would be too much microsoft-like, so no, please.

Bitmaps

Add antialiasing for scaled images. Add an option for "supersapling (highest quality)", "multisampling", and "disable antialiasing (lowest quality, faster)".
HD Image (.wdp) support.