When I try to post an image in the gallery forum I created I get this:

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Looks like you need to check permissions.
GOTO Control Panel » Forum Permissions
Select the gallery forum. The click on the switch button.
There are a couple of settings you may want to check but start with
Total # of gallery images.
You need to assign how many for each group you want to allow.
Next item same page is:
Maximum size of gallery images:
Again for each group but enter the value in bytes.
So if you want 1mb enter 1048576
When done don't for get to click on the update button at the bottom of the screen to save your settings.