Photo editing is one area where the Chromebook lags behind, although if you work primarily with jpeg images for web display the on-line editors are okay. Picasa is a natural, of course, and there's Photoshop Express. The best Chrome photo app I have seen is Pixlr Editor which can work on local images; requires registration, though.
I have now tested a number of photo editors with the Chromebook. Most of them are unable to download the edited picture back to the Chromebook. I found two that were able to upload a photo, do some basic editing, then download it back to the Chromebook. Picnik saves them in JPG format, and Aviary Feather saves them in PNG format. There may be others that work, but those two were the only ones that I found.
What is not supported in Chromebook is Picasa, the downloaded program itself.
What is supported is picasaweb.google.com In fact there is an extension Picasaweb Uploader that will upload your picture files from File Shelf to the Album of your choosing in Picasaweb.