Pictures Appear the Same Size
All your pictures need to be close to the same size. In order to control the size of your images you will need to open them in Fireworks, then select Modify > Canvas > Image Size.
Make sure the Constrain proportions dialog box is checked to make sure the picture keeps its same proportion. This is where you can control the size of all your pictures. You might want to write down each picture size because you will need it for the next section.
Resave it to your folder by selecting File > Export and naming it the same name.