[JWait]

My experience is to make all of the images the same size. You can do this by making the "canvas size" a standard size throughout your site and adding appropriately colored background to fill the unused portion.

Anyway, I think the problem occurs because the "colorbox" is having to render a new size box than the previous one. I don't know if this is "fixable" using JavaScript or not, but I would guess its no.