The difference is the ratio between non square and square pixels.
However this conversion will cause any images or videos with square pixels to look "squashed". Therefore one should use 768x576 images in PAL projects.
If my try to explain more precisely. SD digital standards, in both PAL and NTSC use non square pixels.
- For PAL 4:3 it's 720x576 and upon screening the image is stretched to 768x720, pixel aspect ratio 1.06
- For PAL 16:9 it's 720x576 and upon screening the image is stretched to 1024x576, pixel aspect ratio 1.42
- For NTSC 4:3 it's 720x480 and upon screening the image is squeezed to 648x480, pixel aspect ratio 0.9
- For NTSC 16:9 it's 720x480 and upon screening the image is stretched to 864x480, pixel aspect ratio 1.2
To add still images to video, you start off with the square pixel resolution, in the case of PAL 4:3 that would be 768x576, and you squeeze the image to 720x576 so that it matches the video pixel aspect ratio.