I was literally the last chair in the back of the room at our last Toastmaster meeting and one of the things I noticed was that presenters didn’t use the whole screen for their Power Point pictures. In some cases they even put three smaller pictures together on one screen. This made it very hard to see exactly what was on the screen.
Unfortunately, when people design their PP presentation they are literally 18″ away from the screen and get a distorted view of what people might see who are thirty feet away. What is plainly clearly to you sitting at so close a distance will be difficult for people to decipher who are sitting in the back.
I’d suggest making your pictures as large as possible to fill-up the entire slide. Also crop and adjust the picture so that you only have what you want your audience to pay attention to. If you have three pictures to show, show them one at a time, not all three together.