Another vote here for Pixlr Editor, along with its Chrome extension Pixlr Grabber. That combination means that you can open images in the editor directly from the web (right click on any image and open in Pixlr), or crop your screen directly into the editor. Cuts out the step of downloading and then reopening an image. While Pixlr lacks some functionality, it does have layers and a reasonable range of filters and effects. The main thing that I do miss is that you can currently only left-justify text - no centring or right justification. But for most of what I need to do with images quickly for Roll20, it's fine.