"Gimp", aside from being an open source image editor is also a term used in BDSM fetish communities. I think the joke is basically that the developers who work on the image editor must also be into BDSM.
GIMP
GNU Image Manipulation Program
GNU's Not UNIX Image Manipulation Program (X = "CS")
GNU's Not UNICS Image Manipulation Program
GNU's Not UNiplexed Information Computing System Image Manipulation Program
GNU's Not UNIX is Not UNiplexed Information Computing System Image Manipulation Program
GNU's Not UNiplexed Information Computing System is Not UNiplexed Information Computing System Image Manipulation Program