Though, it's only a warning, not an error, so perhaps the answer is to do nothing and let OXP writers sort it out. If the log entries are getting in the way of other things, set
texture.load.png.warning = no; in
logcontrol.plist to suppress them.
I agree. This is not an oolite, nor libpng problem. Libpng is only detecting a problem with the RGB profile of some images. Libpng is working correctly, and so does oolite. IMHO, the solution is to correct the PNGs profile.
I think that, as soon as libpng1.6 will be in all distros, every new and modified PNG will be corrected.