The part that I think makes this magnet-making method easiest is: I chose some images I wanted to use, sized them down and put some fun frames around them (faux bois, leather, houndstooth), then put them all on one page and printed them out on shipping paper, so they were already on something adhesive. If you wanted the magnets to be glossier, you could print on photo paper or simply use photos you have around, but then you'd need to adhere them to something sturdier, which adds an extra step to these super easy magnets.
Cut the images out of the shipping label paper they were printed on and adhere them to cardboard.
Paint the images with a thin layer of Mod Podge to add some shine and make them a little more substantial. And also because all projects call for Mod Podge!
I have a tendency to go a little Mod Podge crazy, but I exercised restraint and only used two thin layers and it worked out well. Also, painting the images with Mod Podge while they are all on one sheet of cardboard is way easier than cutting them out first then Mod Podge-ing them.