I've made a number of magnets. Here are some things I have done.
On our first cruise we printed our designs using an inkjet printer on card stock, laminated them and glued magnetic strips to the back. They looked ok but we found they kept falling off our door especially when the door opened. Most rooms we saw used the magnetic sheets.
For our next cruise we used the inkjet magnetic paper. The magnetic paper works a lot better on the doors. The paper is pretty expensive so you don't want to make mistakes and you want to maximize use of the sheets. The 2 different papers I have used are Avery (matte) and Staples brand (glossy). You can get Avery for about $1.50 per sheet though
Amazon if you buy 25 sheets. I have only seen the Staples brand in packages of 4 sheets (almost $3 a sheet). I prefer the Staples paper for 2 reasons. I like the colors on the glossy paper and secondly (and more importantly) I have fewer problems. I have a Cannon s9000 printer and for some reason if I have an image with a lot of black in it, the Avery paper seems to gunk up the jets and I get white and black stripes instead of solid black. MAny people have good luck with the Avery paper so it may just be my printer.
If you're printing something that takes an entire page (like Mickey ears to go over your door number) you can just bring up the image for print and print it. Most of mine are smaller than a sheet (I prefer more smaller magnets than just a few big ones) so I use Adobe Acrobat to arrange multiple images on one sheet. You can also use Powerpoint or some other program that allows you to combine multiple images on one sheet.
No matter what you do 1 image or multiple, always do a test print on plain paper before you use the magnetic paper to make sure the images are clear and fit on the paper. Most printers have a setting that lets you size the images so they fit on the page. I usually print with the boarder-less setting.
I've never tried gluing images to magnetic sheets. That sounds like it could work great especially if you laminate.
Have fun.
I don't know if it is because it was an old roll of magnetic/laminate stuff, or it could be because I trimmed the images too close? Anyway, I think I am going to try the printer paper. 
