Wow! Thank you all for your replies and comments!
@Pilou, oganocali, Rich O'Brien: Thank you very much, these resources are spot on. The only thing is I need to take some quality time to read AND understand them...
@solo: Well I have tried this and it looks good but I can't seem to import my textured rock back in Sketchup using Whaat UV Tools...
Edit: I can't even paint in Sculptris today... π
If I understand this, I have 2 options to texture this rock: paint it myself or apply a ready-made texture. In that other case, I'll have to map it and stretched areas in my geometry will be... stretched π