Five to seven seconds is about right unless you do some slow pans that take a while longer. Fun transitions are fine for photo montages. You will want to do a lot of pushes and cross dissolves. Don’t get too fancy unless your audience is very young.
Besides panning, you might find that a very slight rotate is intersting. Start a few degrees to the left and move to the same amount to the right, then the next pic reverse the process. It should be subtle, but easy enough to notice after the first couple of times.