I would like to ask you if there is a way to have the frame margin option, this way the frame doesn't touch the image.
This is not something that Juicebox was designed to do and I do not think that there is likely to be an easy solution. Even if you were to create the border effect you are looking for via CSS on the correct Juicebox class, it would increase the total size of the image slightly and Juicebox would not know about this. The size reserved for the image in the gallery would need to be increased and it may cause issues with the positioning of other gallery elements such as the caption area.
I appreciate you taking the time to post your ideas here but I would encourage you to post them in the Feature Requests forum thread. This is the best place for them to ensure that they are not overlooked by the developers. Thank you.
I also would like to know if there is a way to use a background image in a repeat mode instead of stretch/fill.
Perhaps the best way to achieve this would be to give your gallery a transparent background and set your repeating background image on the gallery container via CSS. (The gallery's own backgroundScale configuration option does not support 'repeat'.)