Here's how I made them!
1: In Photoshop, I resized 9 photos to 3x3 inches & 300 resolution. I made a blank page that was 12x12 and put all the photos on it.
2: Sent to Costco for printing. Cheap!
3. Cut apart photos.
4. Sticky magnet sheets! What a great idea! These were 4x6. Had I realized that, I probably would have made my photos 2x2 instead, to fit more on a sheet.