"If you turn around, I am going to take a photo."
Translation:Si tu te retournes, je vais prendre une photo.
How about using "tourner" (conjugated appropriately with either 2nd person form)? Why would that not work? ...In fact is there any procedure to determine whether to use the re- version of a verb (retourner, revenue, repasser etc), other than when it's obvious that the subject is doing the verb again?
"Tourner" by itself is simply "to turn", for example to the left or right. "To turn around" requires the reflexive "se retourner".
There is no procedure for "re-" that applies to every verb. The verbs you mentioned, for example, have their own quirky definitions not related to their counterparts.
It is because the verb "retourner" means to turn (something) over or around and it only refers to the person, i.e., the subject of the sentence, when it is used reflexively (se retourner). So who/what is turning around, "tu" (or "vous"). Either "Si tu te retournes" or "Si vous vous retournez" are accepted.
I am very surprised that 'to take a photo' actually uses prendre instead of faire. It's the first time I've tried to second quess what the verb might be instead of translating literally and I got it so wrong. I'll keep trying though, faire is used so often, I'm bound to get it right soon. :-)
Probably because there is no que in the main clause. See the following Lawless French lesson that says about the need for a que: https://www.lawlessfrench.com/grammar/subjunctive/