Comment 1 for bug 990799

Do you have gnome-icon-theme and/or gnome-icon-theme-full installed? It sounds like your icon theme is missing some stock icons (similar to bug #785034)

This should be fixed in Pinta 1.3, though, which does a much better job of handling missing icons and not crashing.
(You can test this out in the daily builds PPA - https://launchpad.net/~pinta-maintainers/+archive/pinta-daily)