Dealing with poor projectors
First is to prepare your slides, then arrive at the venue early and test to see what one of the photos looks like. If it does not appear good, try adjusting the Brightness and Contrast settings in PowerPoint for that photo until it looks good. Then quickly change the other photos with the same method so that all photos look the best they can given that projector. The second thought is to create your slides and then make one or two extra copies of the slides. In each of these extra copies, adjust all the pictures the same way. So in copy #1 you would adjust all of the pictures to appear darker for situations where that will make them look better with the projector. In copy #2 you would adjust all the pictures to appear lighter for situations where that will make the pictures look better. Then, when you get to the room, test the three versions (normal coloring, darker coloring and lighter coloring) and use the one that looks the best that day.
If you are in situations where you have no control over the projector, you will have to come prepared with more options to be able to best handle the situation of that day. As you see what works better on a consistent basis, you can refine the options you bring.