going into more detail of what Seth said, the 512x512 is the print itself and is what will be on the print brick ingame. The 64x64 version is the thumbnail, and is what you will see in the print selection menu. The 512x512 goes inside the print folder, and a folder called "thumbs" goes inside the folder too. Inside the "thumbs" soldier, goes the 64x64 image. both images must have the exact same name.