Are you saying that the 600x600 one stretches?
I think it would be good to know what size this digital frame takes, and then resize your photos to that. Let's say your frame takes 1920x1080.
800x600 isn't very large at all (these days). For a 35mm photo, resize it by the larger side, eg if a 35mm photo goes to 1920x1100, then crop the height to 1080.
For a square photo, you have 2 choices:
a) Crop it to 1920x1080 (with your judgement on framing)
b) Resize to the height, 1080x1080, then increase width (image > canvas size) to 1920 so that the image centres (there will be space on left and right side, you can choose to make this black or white or any colour of your choice).
Is my explanation a little too confusing? :P
Leica M6 | Hasselblad 500 C/M | Rolleiflex 2.8