I noticed that a few programs (like kpdf, piklab, pikloops, etc.) were not showing icons in their toolbars. The common element seemed to be that they were all KDE3 programs and I was using KDE4. But that should work.
It took a little detective work, but here’s the solution: I had changed theme files and apparently most these have a missing line that enables this to work. In my case, I was using Nuvola, but the same principle will work with any theme.
1) Find the index.theme file for the theme you are using. In my case, it was in /usr/share/icons/nuvola/index.theme.
2) Edit it (you’ll need to be root — that is use sudo) to save it. Add a single line:
I don’t know that it matters where you put it as long as it is in the [Icon Theme] section. I put it right under DisplayDepth=32.
3) I don’t know if you have to reload the theme, but I did. I just picked another theme, hit Apply, and then picked Nuvolo again and hit apply. You also need to restart any programs that you expect this to fix.
4) Enjoy toolbar buttons with icons.