Hi,
I haven't used Premiere, but since it is a video editing program and since you are importing images for screen display, I would import them at a screen size that is as close as possible to their approximate size on screen (meaning, as they will appear) .. so if you are outputting at say 1024x768, I would be importing images at that size.
Of course, you should working at screen resolution: about 72-96 pixels per inch.
Let me know what you figure out.
Stefan