The pictures are saved to filesystem. after migration, the uploaded path of the images is like "/Content/Images/uploaded/Images/CategoryImages/banners/Markers/Copic/x.png". Nothing change to the source code but database migrated.
During the 3.90 -> 4.00 upgrade the content files in nopCommerce were moved from the ~/Content to the ~/wwwroot directory. That made some of the paths change. For instance, the uploaded images should be in the /images/uploaded rather than the /content/images/uploaded directory.
That makes your file path to be /Images/uploaded/Images/CategoryImages/banners/Markers/Copic/x.png.
Hope that helps!