Ok I know this is probably one of the simpliest questions ever, but it's got me puzzled.

I'm trying to insert a movie clip into a sig, and I followed the tutorial until I got stuck. I press Print Scrn SysRq i know, but everytime I try to paste my screen picture into imageready/photoshop it comes out very small. Therefore I only see a little square of the clip. My screen is set on 800x600 just to let you know. So my problem is, when I paste my screen picture how do I get it so its a larger paste?