A couple of trains of thought on this one and the right answer depends on what you want to achieve and only you know that eh?
1) For performance then make your image the exact dimensions of your video project. Photoshop has presets for these. Also, remember to set the pixel aspect ratio in photoshop to be the same as your Premiere project.
2) Keep the photo as large as possible when imported into Premiere. This will allow the Premiere project to zoom in on the photo and keep all the original detail.
Just rememebr though that if the imiage is different propertions to the premiere project then get the settign sright on the image asset. i.e. on;ly stretch to fit if that's what you really want et c.