The staff in store is OK but Information about preparing images, explaining the difference between high-quality and low quality images, And tutorials would be best for your customers.
Maybe posting tutorials on YouTube and then have a barcode that will lead your customers to those videos. Will help a lot for them before they purchase or send images to be printed.
Keep the video short in a formative will be best.
Something like, Why 300 dpi matter. Pixels versus inches. Contact me for more. Thank