Here is my attempt at a Medieval/Renaissance style lute. Most of the photo in the data set come from Re-enactors/Living Historians/Ren-Faire types, so there are not a lot of lutes that are alike... but it will give you something you can work with.
Since most of the reference photos are people wearing "medieval like garb", it tends to fill in the blanks (that your prompting leaves) with generic medieval style clothes. So if you want something specific, you need to prompt for it, and if you are prompting "out of the box" you may have to weight it pretty decently.
That being said, the sweet spot seems to be a 0.5 weighting... so start there, and adjust accordingly.
"playing lute" is the best prompt for this, as if you have more than one "lute" in the prompt, it might shit the bed.
Good luck!