Aside from GIMP I sometimes use Macromedia Flash MX , because it has some nice properties and tools. (you can cheat and let the computer smooth your lines

this way drawing with a mouse is not impossible for common people)
To save an image in Flash MX : File/Export Image
I think you can download a trial and play with it for a few months at their website
or here
http://www.soft32.com/download_353.html (just sg. I googled now)
Of course if you're naughty you can download non trial versions also...
What I usually do is put in the picture I'm working on on one layer then insert an other layer and draw above the original.
File/Import to library (to get the original)
window/library (then drag and drop)
Insert/Layer (arrange layers in the timeline)
To draw normal lines with the pencil tool set the options to smooth or ink.
Have fun !