I've decided to start working on a personal project, attempting to develop a cross platform, MSPaint like app.  Oddly enough, I find mspaint is one of the applications I miss the most on Linux or OS X, so I want to try to make something similar.  Tuxpaint, mtpaint, gpaint, etc. are all old and inactive and ugly.  I don't want to make GIMP, just the basics, similar in features to MS Paint.
I'm thinking of doing it in python with the pygtk toolkit, but I was interested to hear your suggestions.  Would C/C++ be a better choice, or even C# (gasp!) with mono?  How about using Qt as opposed to GTK, or maybe some other fancy library I don't know about (Please, not FLTK!).  I'd be curious to hear your thoughts.
Thanks!
Short: You can use both, no third party library is guaranteed to be distributed with all major distributions.
Long:
Gtk+ vs. Qt

What do you want incorporate into your application. If it is just selecting a brush, selecting color you could pretty much use any gui toolkit.
If you are going to run it as a web-based tool, Gtk+ has an html5 backend renderer (I don't know about Qt)

A sidenote:
I recommend to use the toolkit's native programming language (gtk+ C, Qt C++) - if you don't, you will suffer from delays with bugfixes, generally more bugs and delayed releases, though for that case it shouldn't really matter.
Everything else boils down to personal preferences and there already exist some questions to tackling that issue.
Framework for paint program