The picture was wonderful. You ought to have seen it.
This feels like an odd use of “müssen”. I would have expected “du hättest es sehen sollen” (you should have seen it). Does “müssen” here strengthen it a bit, like “you really should have seen it”?
You are absolutely right.
" … sollen," sounds much more natural and if I would like to strengthen it, I would rather say “Du hättest es wirklich sehen sollen.”
“Du hättest es sehen müssen” is like “somebody should have forced you to see it” and that does only make sense in rare situations,