Best medium for mixed content - High quality pictures and SD video
It depends on your goal:

If you must deliver to viewers with only a classic DVD player then make it a DVD.
If they only have a Blu-Ray player then make it a Blu-Ray.

For content preservation I would recommend to basically write all original content onto a disk with highest possible quality in wrappers/containers that are widely supported (high-quality JPEG for photos and MPEG4 or MPEG2 for videos). Then most modern players will be able to present the content, as well as allowing you to keep good mezzanine quality.