@Georgia
This is a known issue caused by MobiOne caching a version of the assigned image file in the .mobi file where the image widget resides. The background for this decision is to support portability of .mobi files. But the downside is your scenario where you change the image file on disk.
You need to manually refresh each image. Here’s how:
1) for each image widget who’s image file has changed
a) open the .mobi file that contains the image
b) select the image
c) in the Properties delete the file path to the image
d) in the Properties reassign the file path to the new image file
e) confirm that the image widget is up to date
2) save each changed .mobi file