DPI is irrelevant, use specific pixel size
As with SD DPI setting in HD makes no difference whatsoever. It’s the pixel size that counts. All graphics must be prepared based on the HD standard.
1080 requires 1920×1080 pixel dimensions
720 requires 1280×720 pixel dimensions
If you intend to make “motion control” moves on your still images you will require files larger than the sizes described above. The actual size will depend on how close you want to zoom into the picture.
A simple rule of thumb is to imagine an invisible 16:9 frame around the area that you will be zooming into. The pixel count within this frame should be close to the required size appropriate for your standard.