Using this you still can't make a dimension "in air". You must select some edge or feature.
Additionally, this reference dimension is not available in drawings.
I came to SolidWorks via KeyCreator/Cadkey and there I could make a dimension anywhere. Even "in air". The dimension would have a prominent red line under the dimension indicating that it is not associated to any geometry. (the red line would not print). When you have the ability to use a function, you may be surprised how often you use it, and you miss the ability when your new CAD software can't do the same trick.
The only option is points or other lines/arcs/etc. Not as convenient as it could be...
I obviously hadn't read the question properly, I missed the bit about the dimension going onto a picture.
Teach me to answer questions in a hurry!
However you can bring model reference dimensions onto your drawing. You can use the insert model items tool to do so.
Works great! Thanks for the info!
Ill work more with points, as im discovering they dont print so i can use to reference anything!