Got it, I will keep this option in my back pocket for those few occasions when I know that a map or image has changed between sessions.
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Another possible option with FGU is to add your updated image to the campaign as a new image (e.g. Map_v1, Map_v2) and add the updated map as a new layer to the existing image. That might get around the issue without having to have the player's clear their cached version. I've started using this method for a map I update between sessions and it's worked so far.
This is the habit I got into with FGC - always tweaking the name of an updated image with something like v1, v2 + stop sharing the original, share the updated version (and even deleting the old one).
Would asking to have image cache check file date stamps as part of the update process be a viable wishlist item (or too complicated/too marginal to consider)?
There are several ways to approach this. It is really up to you as to what suits your needs. I am not familiar with the software you are using, so on that point, I can not advise but simply saving the new map as a slightly different name. For example, map_1 as your original and map_2 as the updated version will solve your issues. Then you may add the new image as its own entry into the image and maps window or, as previously stated, as a layer on top of your original.
I also stream FGU image related tutorials on our twitch channel every Saturday at 3 pm Eastern time. I am more than happy to demonstrate how to deal with any issues you may encounter and explain all of the workings of the imaging system.