Probably the best freeware for tasks like image editing is GIMP. You can even use it to create custom 3rd-party themes for Windows 7. Here’s how you would take some screenshots in GIMP

Gimp Grafics

There is a Windows version of GIMP wich makes the task of taking screenshots and saving them really easy.

Gimp for windows

(1) Use the ALT + PrintScreen option to take a screenshot of the current window.

Gimp aquire Menue

(2) “Paste as new” will create a new image document and automatically paste the content of the clipboard