You can use easy graphic converter, here:
http://www.etrusoft.com/graphic-converter/
It can convert and resize a mass amount of images at a time, so you don't have to do it yourself. (of course, if you're using printscreen, you need to paste it in paint and save it as a png or something, then convert it to a jpg when uploading. Paint saves jpg's at very low quality and is not recommended unless you want the file to be very small)