To ensure that your exported work is high quality, Memento Composite displays resolution warnings for low quality images on your layouts.
DPI warning and errors will usually occur when increasing the size of an image (such as using it on the canvas background) or from adding low-res graphics downloaded from the internet to your layout.
Occasionally your portraits may trigger a warning if they were exported as very small files, suitable for providing to the school for very small yearbook photos or preview images.
Warnings on backgrounds can usually be ignored, since backgrounds are usually textures, not high-focus images. Be more careful with logos and clipart, as they may appear poorly on your exported files. Either resize the image so it is smaller on your layout (the error or warning may disappear), or replace it with a higher quality image.
At first launch, Memento Composite will ask you to confirm what dpi warnings and errors you would like to use. You can enter custom values or use the suggested values (100 dpi to show a warning, 50 dpi to show an error).
To change the dpi warnings to a new value, click on General in the top left corner, and choose User Preferences.
There you will find the DPI warnings settings, which can be customized.