Under most circumstances you will want to use images that are at least 200 pixels per inch (ppi) relative to the size of the product being ordered.
For example, you will want to use images that are at least 800 x 1200 pixels in size for 4x6 prints or images of that size in any other product. However, there are exceptions to this rule. Enlargements may still look good with a lower resolution measurement (in pixels per inch) because the photo quality will not usually be scrutinized closely. You will still need to provide a high resolution file, at least 1500 x 1000 pixels for anything 8x12 or larger, including our larger and medium sized products (i.e. Pillowcases, Woven Throws, Cutting Boards, etc.).
Whether you need a photo with even higher resolution to ensure good quality will depend on the subject matter. Some subjects demand more detail than others and looking at an enlargement up close will expose any lack of detail in your image if the resolution is low.