How can I make the best quality SD DVD's
There isn't one.  What you have been told in the past is correct, DVD resolution is very, very limited.  720 by 480 is barely even 1/3 of a megapixel of resolution, which is exceptionally low and will look poor on pretty much any HD display.  There is no solution for going higher quality than this on a DVD that is playable on normal DVD players.

The reason that it is not all that noticeable when you watch a video on DVD is that the information presented over time is not all the same and thus your brain is able to piece together a picture of what things would look like based on successive frames of video.  This makes it seem smoother and clearer than it actually is, but it relies on the content moving.  Static images present no new information in successive frames.

It is possible that you are using a poor quality encoding or too low of a bitrate, but if you are getting very similar results from both programs, it sounds like it is probably just the limitation of SD formats.  (A poor quality encoder or too low of bitrate would result in artifacts being introduced to the image, but photos should require very little bitrate since they are static.)  If you can post a screenshot captured from playing back the DVD on your computer, that might help us better evaluate, but my best guess is that you are just noticing how poor quality DVD actually is.