image

800 by 600 is probably the ideal size for images. Anything larger and you'd have to scroll to see part of the image not shown on a given screen.

tsar, maybe poster is asking about their avatar?

What's good about Gravatar (I think the limit for an image file size is 5mb not pixel resolution) is that it scales down the image (as long as you crop it into a square).

For me 900 is the largest that will work, and that's for the width.
I tend to go for 900w, 600 length if it's a horizontal rectangle..
rarely go much below 500.

There's a how t0 here:
http://blog.able2know.org/2008/09/13/new-image-bbcode-features-size-and-align/