Picmonkey is a brilliant online editing site. You don't need to download any software, it's not one of those dreadful "we're going to trawl through your whole computer now" programs and most functions are free. NONE of our tutorials require the paid upgrade.
You can crop, resize, add special effects, text, stickers etc and even give yourself a makeover.
It's fairly intuitive and you can practice with their example images but we've prepared tutorials for you:
Saving Images in Picmonkey
Resizing Layouts for Tablet, Smart Phone etc
Separating Alpha Sheets
Recolouring PNGs (transparent embellishment files)
Adjusting a Background