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
Although, it might be better to make the monsters really subtle, so it looks like a generic sofa-sized painting at first, but on examination...
He's not the only one doing this. Cool stuff.
http://www.etsy.com/listing/69786953/sea-monster-acrylic-painting
1 bad painting +
More bad painting =
Really Bad Painting!