I make personalised story books for children. My books use a photo of the child's face inserted into the illustration to make them the main character in the story.
When my customers send me the photo of their child, I try to explain to send me a hi-res photo and not to download from social media, but often they send very low-quality photos, which will not be accepted by my printer who needs at least 300DPI for these 8 x 10 inch books (the child's face is around 3x4cm when printed).
I could get my website to only allow a certain filesize, but I know that isn't always a true measure of whether the photo will look good. For example if the photo is a close up of the child's face, it can be a smaller filesize than where the child is in the background of the photo.
Can anyone explain the parameters I need to consider to make sure the face prints correctly (especially if I were to have this coded so my order form would check the image's suitability), and how best to explain this to my customers?