It's not quite that simple ...
You have two issues that need attention, the "gap" around the picture for easy insertion and expansion of the frame AND the depth of the rabbet.
The OP spoke of a 6 mm rabbet. If you need a 2 mm gap all around the picture, that leaves 4 mm of frame to secure the picture. If you have a 1 mm gap all around, that leaves 5 mm to secure the picture.
Other issues are whether there is a glass in front of the photograph/picture or is there a mat like around a water color painting, or is this an oil painting with no glass? All these need to be taken into account when framing artwork or photographs.
This is why I like working with a story stick or an actual length stick to see exactly what's happening and where to make my mark for the remaining miters. I like to leave an extra 1/4" or so to trim away for the final miter dimension.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)