There are many Slide Styles out there that just work best when all the photos you are using are the exact same size. The 10 Flip Up and Down Slide Styles and Borrowed Angels Template are perfect examples. With different sized photos, the 10 Flip Up and Down styles would look a bit of a mess. With the Borrowed Angels Template, making your photos the same size as the mask makes for a much easier show build.
I'm going to show you a quick way to make your photos the exact same size (without squishing anything) in Photoshop CS5. The same methods can be applied in just about any other photo editing program.
First, open your photos in Photoshop:
You can see, these two photos aren't the same size. To find out exactly what their sizes are, click on Image>Image Size:
I've done this with both of my images so you can see their dimensions side by side:
The photo of the girl in the red dress is 1440 x 2076, which is pretty large! The black and white photo of the hands is 646 x 778, which is small for just about any purpose, except for what we're doing! Remember, the dimensions of your slide show are around 720 pixels wide, so this photo will almost fill the screen. For the most part, you'll want to scale your larger photo down, NOT your smaller photo up. That will give you some nasty pixelization.
So, I'm going to resize my larger photo to something close to my smaller photo....NOT being exact. I'm just going to type 850 into my Height number box and let the Width number box fall where it may (make sure you have "Constrain Proportions" checked).
This is what I see when I've changed the dimensions. Don't panic...they aren't really that different in size. We're viewing the Black and White photo at 100% and the Red Dress photo at 33%. Now, with my Move Tool Selected, I'll just click on the Red Dress Photo and drag it right on top of my Black and White Photo:
As you can see, it still doesn't quite fit. That's ok. These two photos weren't the exact same shape to begin with: one was a taller rectangle than the other. We don't want to "squish" our taller photo just to make it fit inside our smaller one. We'll have to resize a little. In Photoshop, hit Ctl + T to get the Transform Bounding Box:
While holding down your Shift Key to constrain the proportions, click and drag the corners until your photo fills the frame.
You now have a photo that is the same size as the canvas it's sitting in. From here, make sure you save this as a new jpg. You don't want to write over your original. Then bring this new image and the original black and white photo into ProShow and they'll be the same size (which means they'll behave the exact same way. Aaaahhhhhhh.....)