Another vote for Paint.NET for me - a huge range of useful addons and the GUI is about as simple as this sort of program can get. However its not so good at drawing anything but fairly simple stuff from scratch, at least not in my hands. My avatar icon here was created entirely using Paint.NET.
Where it excels IMHO is for adding text, effects, enhancing, manipulating, editing, combining, correcting, re-sizing etc existing pictures. A key features for me is the ability to use multiple layers.
One of the other posters also mentioned IrfanView. I've had this installed for years and barely touched it once I'd found Paint.NET. But recently I've used it quite a bit (pun intended) as I needed to reduce the file size of some icons for use with a games' console but maintain maximum PQ.
The menu based controls make it feel clunky after using Paint.NET but, whilst certainly not comprehensive, it has an impressive set of features and can do other things that a pure graphics program like Paint.NET can't.
When you install the addons packs it gives it a greater range of save file types and image quality options than even Paint.NET, which itself isn't lacking in this respect either.
In all likelihood with these two programs available, excepting specific platforms and professional needs, they'll make any other programs for converting image file types unnecessary.