I use heavily toned-down background images for all my chat windows, though mIRC likes to seemingly eat them at random; I'll change channel focus, and the backgrounds all spontaneously disappear. The same thing can happen if I switch focus to another program and jump back.
I know for damn sure that it isn't a matter of resources, as I have two gigs of physical memory before even dragging the pagefile into the picture, and yet it DOES seem to tie into how much RAM mIRC is occupying at the time. If I close a few channel windows, the backgrounds magically reappear on their own, until I switch to another channel, at which point they disappear.
I've noticed this issue coming up ever since 6.15, as my older versions of mIRC don't seem to ever be affected; if I were to start up another instance of 6.17, it would open with no backgrounds, including the side- and titlebars, until I close the initial instance of 6.17 that ate the backgrounds. However, I always have a seperate client window open in 6.12 as a fileserver, also with backgrounds, and it never chokes.