How can I tell if the photos I purchase have transparent backgrounds and / or have clipping masks?
Well images on photodune are supposed to be uploaded in JPEG as far as I know. JPEG doesn’t support transparent backgrounds as such. As for clipping masks I assume that it’s the same.
You can always photoshop it to take out the background making it transparent.
Hope that helps
QuickMedia is correct—all images on PhotoDune are submitted in JPEG format, so they cannot have transparent backgrounds. They may have clipping paths though, as that can be saved even in a JPEG file. Authors whose images have clipping paths will generally include that information in their tags or description. Try adding ‘clipping path’ to your search terms, or contacting an author to verify that the image has a clipping path before you purchase.
Alternately, GraphicRiver has a category called Isolated Objects:http://graphicriver.net/category/isolated-objects
As files on GraphicRiver can be layered images or sold in packs, you may find the item you’re looking for in that category with a transparent background.