The API gives me a URL to retrieve the image at. What image formats does SmugMug use? Will it ever give me a raw format, for example? Thanks.
SmugMug supports JPG, PNG and GIF photos. In addition, with the Power and Pro account you can also upload videos, which are converted to MP4. So these should be the file extensions you that you may encounter.
There's also the additional SmugVault service which allows you to upload any kind of file type that's usually not supported on SmugMug. I'm no expert on the API and am not sure yet if the API also allows downloading of SmugVault files, but if it does, you may encounter any type of file format, including RAW files, depending on what you upload to your SmugVault.
Maybe someone more familiar with the API, can jump in to provide more details.
For reference, here's the link to the current API documentation.