Chris McMahon calls them "involuntary collaborations". He buys fairly dull landscapes at yardsales and then adds monsters to them. To give credit to the first artist, he just adds his own name in the bottom right corners. You can view two others at the link.
Link via Geekosystem | deviantART Profile
1 bad painting +
More bad painting =
Really Bad Painting!
http://www.etsy.com/listing/69786953/sea-monster-acrylic-painting