Description of problem: After updating my system today via yum, with libpng10 on the list, I had to reboot to kickstart a flaky sound card, and when the system entered runlevel 5, the Bluecurve login screen was no longer recognized ("png is not a recognized format" was the message, I believe). I got the bare-bones login window instead. After logging in, all png icons on the desktop and in the gnome panels (including menus) are gone, replaced by nothing (no icon, just grey space), or a large red "x"). When I tried to change the desktop theme, most of the installed themes (the ones using png elements) no longer work (same error). However, the desktop and panel icons that weren't displaying properly changed from the red "x" to the generic gnome footprint icon. When trying to change desktop or panel icons (right click, then Properties), all the png's in /usr/share/pixmaps are not seen in the available icons (though they are still in the directory if you check with a terminal window). Version-Release number of selected component (if applicable): libpng10.i386 1.0.21-1.fc6 How reproducible: Every time without fail. Steps to Reproduce: Just run startx if in runlevel 3, or boot to runlevel 5. I assume this is the same for other levels except 0 and 6, of course. Actual results: See description above. Expected results: The ability to see the normal png's on the desktop, menus and gnome panels. Additional info: One thing I noticed was that /usr and /usr/share/pixmaps permissions were changed (no write permission for user, group or all). This was not the case prior to update. Adding write for user (chmod u+w) on these diretories had no effect. Also, my wife keeps jpg's on her desktop, with gnome displaying smaller versions of the photos. Some of these small versions displayed okay, but some would not display at all, reverting to a generic icon instead.
I don't know what's causing this but it shouldn't be libpng10 since that's only present to support legacy applications built against ancient libpng versions; the GNOME and KDE desktops will be built against the modern libpng package and will use that to handle display of PNG files. You might try "yum remove libpng10" to see if there is anything on your system that actually still requires libpng10 (if yum wants to remove some application you use, just reply "n" when prompted to confirm the removal). It's also worth checking that your libpng package is OK, using "rpm -V libpng" (this should produce no output if everything is OK). Another useful check would be to try "package-cleanup --problems" to report on any dependency issues in your system's RPM database; the "package-cleanup" program is part of the "yum-utils" package.
Thanks for the pointers. It did end up being a munged libpng/gdm problem, which was easily fixed by reinstalling the proper update packages.