No, this is not an unfortunate slur - it's actually an acronym for "GNU Image Manipulation Program."* And what a program it is! GIMP is one of the most mature and functional open source programs out there, and should be the first choice for anyone looking to do any sort of image editing.
GIMP is available for Windows, the Mac OS, and Linux. The closest commercial equivalent to GIMP is Adobe Photoshop - just like Photoshop, GIMP excels at editing raster images. It is under very active development, and new versions and bug fixes are constantly on their way.
As a tool for editing photographs destined for publications or presentations, you really can't beat GIMP. It has a whole host of very functional tools for selection, touch-up, and flat-out manipulation of images. Want to rotate a portion of the image? Easy enough. Need to remove a black background and replace it with white? No problem. There's not much more I can say - GIMP is fantastic! To be perfectly honest, I haven't missed Photoshop at all since making the switch (although I am sure Photoshop "power users" might disagree).
So are there any downsides to GIMP? Some users may report slow speeds, but this seems to be largely fixed in the more recent versions. For folks who may want to do extremely hard-core editing of color images for later printing, GIMP only supports RGB color formatting (although you can choose colors on the palette using CMYK standards). This may pose a problem if you want to send your files to a professional printer, but it should not affect the average user (or the paleontologist who is usually working in grayscale images). Finally, the GIMP toolbar and image editing pane open as two separate and discrete windows, rather than as subwindows within a main window (as in versions of Photoshop that I've used). This sometimes creates a cluttered editing experience, but it's more an annoyance than anything. Integration with a tablet can be a little bit of a hassle, but it works pretty flawlessly once you get it running (and I've never tried the same task in Photoshop, so I don't know how it compares in that regard).
*GNU = a type of open source software license; it has nothing to do with the African savannah.