Memento Yearbook is designed to warn you if an image on your page isn't sufficiently good quality to print at the current size. Images that do not meet your yearbook company's recommended quality will show yellow warnings or red errors.
How resolution warnings are triggered
How to avoid low quality images
Only use original photos taken on a camera or a phone set to high quality images. Should you have access to a collection of backgrounds that you are legally able to use in your yearbook without copyright violation, look for a resolution of 2700x3450 (8.75x11.25" @ 300 dpi) to cover a standard page background (including trim) without triggering any resolution warning.
Images cannot be "up-sized" to fix resolution issues. If you download a photo then open it in a photo editing software, you may be able to increase the size of the photo, but the image will still print blurry (without a warning) because it is not possible to increase the amount of information/detail in the image.
How to fix resolution warnings on your page
If you see a yellow resolution warning on an image, try reducing the size of the photo on your page, or, if possible, zoom out of the photo. Replace or remove images with a red warning on them since these will be particularly bad quality when printed.
Images used on your canvas background are more likely to display these warnings, as the photo is being stretched to a larger size. Zooming in on a photo placed on the background may trigger a warning.
If you find that your background has a yellow warning, check your proof carefully to ensure that the canvas image is sufficiently good to be printed. Generally background canvas images do not need to be as laser-sharp as photos on your page, as they are more textural/atmospheric than precise. Red warnings should be respected - find a new background to use.
If your portrait photos are being flagged as low resolution, contact your photographer. They may need to provide you with larger images that follow the recommended resolution of the Professional School Photographers' Association (640x800).