IrfanPaint is a freeware plug-in that adds painting features to IrfanView. It is being developed by Matteo Italia (in collaboration with Irfan Skiljan). This plugin requires a modified IrfanView exe (that is included with the plugin) "since it must include a call to the dll of the plugin". I just renamed the original exe and extracted the zip-file and it works well so far, given the case that it is still in beta..
btw, to invoke the plug-in, load an image in IrfanView and press F12.