Picnik, the popular (and free) online photo editing application is great for those times when you need to make some quick edits to an image without launching Photoshop. Picnik also works well with images you already store online, they have integration with popular sites like flickr, facebook, Picasa and photobucket.

Checkout this video tutorial from flickr user Torley on using the Picnik add-on for Firefox to capture and edit screenshots with great results.

Get the Picnik add-on here