This will work for still images: Export a still frame of the image you want in Premiere and open it in Photoshop, magic wand the background and delete it, save the file as .psd, and then import the psd into Premiere. In Premiere there are layers that you can stack on top of each other – video 1, video 2, … The layer that is on top will show up over the other layers. Place the psd on a layer above your existing video and the still image will play with a video behind it (showing through the background that you deleted). As far as deleting the background of a whole video you might have to do these steps for each frame (30 frames per second).