Gimp 2.6 help pleeeeeease?

Alright, i am going to give as much info as possible because I need a SIMPLE yet THOROUGH answer! i have gimp 2.6 and i am trying to make a background for my twitter account. i need to have a picture of a celebrity that I like included in the background. i open up gimp, and get a blank slate (if anybody knows how big it's supposed to be so i can successfully upload it to twitter, that would help too). i copy a picture of the celebrity from the internet, then i go to paste it into gimp and it says 'there is no data in the clipboard' what does that mean?? how do i copy and paste to gimp in order to make a simple twitter bg? again, please keep answers simple yet thorough! thank you :)