Resolution refers to the number of dots per inch (dpi), or the amount of detail the image has.
Higher resolution means a more detailed image, and also larger file and longer upload time

Your images should be scanned at 300 dpi and your files should be created at 300dpi at

THE FINAL DIMENSIONS YOU INTEND TO USE THEM.

Please note:  Simply increasing the DPI setting in a graphic program does not improve the resolution.