I have my own script to grab the max viewport bigger than screen size, and it is very useful as you can get higher res grabs of viewport shader effects which may take time to setup render materials for. :D
Picasa, Imagru, Flicker and many others are what works. Also, depends, if you're Viewport capturing from Max and Maya packages, you can get scripts to do the job for you and/or use Grab Viewport under tools sections. You could also render...
When I use this script to make a screenshot, I also make a copy of the material and set all the diffuse components to fully self-illuminated white, and make the background black. You can then just swap between the two materials whenever you need to re-render. You can also just render a proper alpha, using the render…