The usual reasons for image frames losing their images it that either the document is moved to a different folder or the images have moved (or the folder they're in has been moved or renamed, or the image has been renamed).
Scribus doesn't keep images with the document - it doesn't import them - rather, it maintains a "relative path with filename" to them on the storage device so if anything moves - or the image file is renamed - the link is broken.
For example, if you have:
- Home
--- Documents
----- newsletter.sla
--- Images
----- award.jpg
--- Archive
And - between closing the document and opening it again - you move either newsletter.sla or award.jpg to another folder (e.g. Archive), or you rename award.jpg to something else, the "link" between them will have gone and Scribus won't be able to find the file. (The same problem can also apply if you use removable media.)
P.S. By not importing the images into the Scribus file it helps to keep the document size down by quite a lot. It also lets you edit an image "on the fly" without needing to re-import the new version.