If the image is from a game or video the app that is running is using something called DirectX and printscreen may fail.
There are workarounds such as lowering the video acceleration but gamers are often unable to do that. For a video you should use the video player's own snapshot feature.
Again, all well discussed and I will not duplicate why here but note the prior discussions and offer a link.
http://labnol.blogspot.com/2005/11/capture-still-images-from-windows.html explains this again.
Bob
Hey guys. I have a problem when i use screen
capture into any application, when i paste it there are a lot of black
bars going through the image. Around 15-20. It kind of seems like Vista
has an option preventing me from print screening
any image. This also happens
when i try to capture video
footage on my computer using fraps or any other video program. The bars
look exactly like when Vista asks me for administration rights to use a
program. I have never been able to take a screenshot because of this.
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic