What is GIMP and how do I get a copy?

More about GIMP that is not covered in the link above.

How do I use GIMP to stretch width while keeping height the same.  (I use this for my Facebook page cover.)

How do I crop a circle out of a layer?

I just downloaded a free copy of GIMP.  Now I have some very annoying new popup advertising on my Mac.  How do I get rid of it?