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.
Add color management support. Need the ability to not only recognize embedded ICC tags, but to choose a default profile to "assume" if image doesn't contain embedded profile tag. (This is an emerging problem with the recent release of monitors displaying wider gamut than sRGB -- eg. Dell 2407WFP-HC -- as they display non-colour managed images more colourful than they should appear.)

Bitmaps

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