We recommend that all artwork be designed at full finish size with images being at 150 DPI.
In some cases, templates may be designed at half size in order to be compatible with Photoshop and Illustrator artboard limitations. When templates are at half size the DPI of your images needs to be 300.
A good way to double check the DPI of your embedded images is to zoom in to your full size artwork at 100% and half scale artwork at 200%.
For more details on setting up your files properly, check out our Art Guidelines.