I thought it was main_icons_sprite_hd.png but I'm having no luck. Changed desktop icons for Forum (the colored F), the checkmark for Rules and a new calendar icon lickety split. Can't be cookies. I cleaned those suckers like I have OCD. I'm guessing I need to set the coordinates even though I replaced the exact icons I want to substitute for? But I don't see where to do that.... Lost!
Images are cached. It's not browser cookies you need to clear, it's the browser cache. ;)
Alternatively, use Ctrl+F5 on desktop. Same result.
Oh, well, that's not the problem either. I just used the wrong term...
Solved! I didn't understand the coordinate system correctly but after reading the css a dozen times it became clear I had things in the wrong order. Peace