The easiest I've found (at this point) to use partly prefabricated characters to easily make a strip has been to use an online comic generator.
The options change from year to year, but a search for something like "online comic creator" should get you a number of candidates.
However, Papuix's recommendation for TBO sounded good, so I decided to look at it. That link was dead, and the best link for TBO that I can find to a third-party ppa: http://www.ubuntuupdates.org/ppa/getdeb_apps?dist=precise
It does seem to work in 12.04. The tutorial is in Spanish, and oddly, it consists of just page after page of text without images.
TBO works like this: you create a frame (i.e., rectangle) of some size with the frame tool, then double-click it. From there, you are now able to use the formerly grayed-out icons to add (stock) characters, text bubbles, text, and so on. There are a few included characters and some basic character outlines with a few different mouths, eyes, etc.. It's a bit limited. I had some fun with it, but you'd have to import your own art to give it more than a very limited scope.
It made me think of 'potato guy' aka
ktuberling, which is, I suppose, intended for kids, but is conceptually not much different. It also has a few included backgrounds (e.g, Egypt, the moon, a robot space station), and probably more available characters (e.g., the robot pieces can be assembled various ways) than TBO. It doesn't have the generic set up of frames, though, which is a decent feature if you are actually doing a strip-like comic.
Other than these, I'd really just recommend importing art into
inkscape (or your favorite paint/drawing program) and going from there. Or even starting with some basic shapes in those programs, then saving and re-using them. This would be less limiting, and not much more work, than trying to use some of the partially prefab options that are currently available.