Capturing Images

OK, I'm going to try and learn a new and useful skill.  If someone out there is willing to teach it me.

I want to capture pictures and use them to make icons, even when the pictures aren't easily captured.

I have a Mac.  I know about Grab.  I used it to capture a Saturday Evening Post picture I'm particularly fond of.  The app behaved like a little lamb, producing a lovely TIFF file.  Which won't upload on LJ to make a brand new icon for me.  Why not?  Did I do something wrong?  Are there other ways of saving the image?  Am I going at this all wrong?  Should I go back to carving icons on primordial slate or leeching off my more up-to-date friends on a regular basis?

For the answer to these questions, stay tuned to this channel.  Or wait until I've had time to write one of the posts I've had in mind for when I have time to write it.