What program do you use to group pictures together?

Hello, I just took some before pictures (front, side and back) and I want to put them all side by side and print it out and put it up on my fridge. I have used paint to do that before and also photobucket but they always end up a different size when Im done which makes it hard to print them and them be clear. I have seen tons of people with their before and after pictures side by side (as one picture) and would like to know what program yall use to do this. I am not the smartest person when it comes to computers so it cant be anything supper tough (: Thank you very much in advance for any advice.