The Gimp is a fully featured graphics editor on a par with packages costing hundreds of dollars/pounds. Developed for use on Linux, there is a Windows based installer. The power of this application cannot be underestimated. I have been using it for many years and the depth and range of features has never let me down.
Gadwin Screen Capture is a great little utility for capturing screen shots from your computer. Once installed you can take pictures of your desktop or of specific windows/applications on your desktop.