Combining two or more photos into a single photo is actually very simple. Imagine having actual prints of your photos and say they are 8 inches width x 10 inches height in size. Then imagine laying out a piece of paper large enough to hold each print. If you wanted to place the photos side by side, you would need a sheet of paper that would be twice as wide as your prints, so you would need a sheet 16 inches wide by 10 inches high. If you wanted to place the photos on top of each other, you would need a sheet of paper twice as high as your photos, so that would be a piece of paper 20 inches high and 8 inches wide.
You will do the same thing in the computer. In Photoshop or GIMP http://www.gimp.org you will go to FILE - NEW and enter the dimensions of you new "canvas size". You can either do this in inches or pixels or any other unit of measure. There is a drop down list beside the size boxes where you can select units of measure. Again, you will create a new canvas that is large enough to contain your photos you want to place into it. You will need to know the exact size of your photos. You can go to IMAGE - SIZE and see the dimensions of your photos and then from those numbers you will know how large to make your new canvas.
Be SURE, however, when you are creating your new canvas, that you set the resolution (DPI) to the same number as your original photos, otherwise there will be a huge size difference between your new canvas and the photos you drop into it.
You can also make the new canvas larger than your photos if you want to add borders or a different color background or text or design embellishment. Only your skill and imagination are the limiting factors as to what you can do.
steve