You can do both with existing images, as well. Set your container field to External storage (in Manage Database > Options > Storage). This will shunt the images out of your database and into an external directory (whether you're on FileMaker Server or not). The size of your database will then shrink considerably.
FileMaker will automatically create thumbnails for you for any images in container fields (including those stored externally), if you turn this capability on. Choose File > Manage > Containers and select the Thumbnails tab.
Worked like a charm.
- I created folders with the field names and after I changed the containers to external, it ripped the images out and put them in the external folders.
- I changed the container storage settings, by using "Manage Database".