Quick question

Call me stupid, but I'm having a problem with Adobe Photoshop (it came in a Create Suite package, if that answers any other questions). I had been using PSP9 for almost a year prior to this, and I like to think that I'm not a beginner.

However, for some reason, I find myself unable to copy and paste images into Photoshop. I copy the image, go to paste it, and nothing shows up. I try to select Edit > Paste, but that option is grey, implying that I haven't copied anything, when I know I have.

Help, anyone?

Thanks in advance!