Finding artists is much easier for open source games that based off known titles because there is people who like original game or already participate modding community. Many open source games even started this way (like The Dark Mod).
Personally I'm participate development of VCMI (FOSS engine for Heroes of Might & Magic 3) and there is plenty of artists that contributed to modding community in past and totally okay with contributing their work under CC licenses. Sure that for more popular titles it's may be even easier.
But I agree it's really tricky to find contributors who going to work on brand new game or professional software like GIMP.
Personally I'm participate development of VCMI (FOSS engine for Heroes of Might & Magic 3) and there is plenty of artists that contributed to modding community in past and totally okay with contributing their work under CC licenses. Sure that for more popular titles it's may be even easier.
But I agree it's really tricky to find contributors who going to work on brand new game or professional software like GIMP.