Problem with video output on Mini G4

My mac mini G4 (1.42 GHz), which was working fine before, now display video only during the gray screen at startup... after that, the screen goes black. The problem started when i changed the desktop background pic for a new one... The video output became distorted and the screen turned black shortly after.

I tried resetting the PRAM and PMU, as i've read here, but that doesn't change anything.

Thanks for any help!