Basically an image used in your document should be large enough to support the size of the frame of the image on your document and the crop of the image when respecting the resolution (DPI) of the document.
Several factors influence this situation:
An image with insufficient resolution may appear blurry or show rendering artifacts (e.g. thin white lines) on your exported document.
Posterino will notify you in cases where it finds images with insufficient resolution when exporting your document.
You can handle a resolution warning in several ways:
If you can life with a blurry image or some artifacts on your document, just ignore the warning and proceed with the export of your document. You may like to inspect the exported document with the Preview application to verify that the result is satisfying.
Reduce the DPI or size of your document
Reduce the crop of the image in question
Posterino can display a small "resolution warning" symbol on the bottom right corner of your images: 
. This warning sign will appear every time the size of an image frame or the crop of an image can't be supported by the size of the image. Right when editing the image.
You can control this feature with the checkbox under > > > .