At a recent TA PD session we used Pixlr (like Google Docs, but for images) to create color splash images like the one below:
The larger point is that Pixlr (http://pixlr.com/editor) is a handy tool to do simple edits like cropping and resizes up to intermediate ones like manipulating color, adding effects, and working with layers. Plus, it works in your web browser so there’s no need to download anything!