The following are recommendations on obtaining the best performance from physical target images. Image Targets should be rigid, not flexible, with matte surface, but no gloss. Small targets are good for user manipulation. Be creative when choosing targets; make them appropriate, relevant, and fun.
A hard material such as card stock, plastic, or paper fixed to a non-flexible surface is better than a simple printed piece of paper. The reason is that the flexibility of the printed piece of paper can make it difficult for the object to stay in focus. However, paper targets are easily reproducible and widely available, so do not completely discount them as valid targets.
Be aware that even if you provide detailed instructions for the printing size and the paper quality, most users will revert to the default of their printer, which is usually A4 or US letter. To avoid these printing problems, you should provide targets that are books, marketing materials, packaging, or posters.