Open an image editing program and crop the image to that size. I do not know which program you have, but on Windows “Paint” (Start -> Programs -> Accessories -> Paint) will do. Assuming the image is too large open the image in Paint, choose the “rectangle” tool (the rectangle with the dotted lines around it) and then select the area. You will know the size from the numbers in the bottom right hand corner of the paint window.
Once you have the right area selected, choose “copy” by right click then do “file -> new” and then paste.
Save the image in the appropriate format (jpg will do) and then upload as above.