The simplest way is to upload them and use our star rating system to check their quality.
However, if you would like to check your images on your own computer, one rough guide is the size of the file. If the image is less than 50K it is probably not suitable for printing. An image between 50-100K large should be O.K. for one panel of the wallet, but generally speaking it’s probably best that your images are all at least 100K.
You can also use many photo editing programs to check image size and quality, including dots per inch (DPI) and pixels per inch.