Sizing digital still photos for video?

I hope this is in the right section. I've searched all over and still get the answer I need.

I am creating a video in MPEG-2 and for videotape that includes still photos from my digital camera. The video from my DV camcorder is fine, but the still photos come out a bit pixelated. I can reduce this problem by keeping the image size (height and width) larger and the pixels per inch larger but it still bugs me. I have a feeling it has to do with the aspect ratio of the video to the still images. Does anyone know the proper width and height for still images in video?
 
Best is to keep them at 720x576 (PAL) or 720x480 (NTSC). ie the same dimensions of the DVD picture. Anything smaller can be distorted/pixelated when finally shown on the TV.
 
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