WebDirect does not manage a cache for images stored in a field. Each time the web page is refreshed, it generates a new path. This increases uselessly WebDirect engine or FMS to generate each time a copy of the image on the hard drive. And as well the browser has to reload every images. I think WebDirect should manage a cache for container fields, even with multiple users connected to the same database. I noticed as well that the thumbnails (when you set the field to manage thumbnails) are also recreated every times.
HTML code produced for the same image :
How to replicate
With an image stored in a table container field, create a model with this field. See file test.