We've figured out what is going on here.
Once upon a time mojoPortal defaulted to allow admins to use the full directory structure under the "Data\Sites\[n]" folder of an install. A while back this was changed to default where admins had to use the "data\sites\[n]\media" folder. There is a user.config setting to control whether or not admins can use the data folder. It is named "ForceAdminsToUseMediaFolder".
Our BetterImageGallery module doesn't take that setting into account. We're fixing it for the next version.
If you have access to the command line of the server your site is on, you can use the "mklink" command to create a symbolic link that will fix the problem. To do this, open command prompt as an administrator, change directory to "<path of your site>\data\sites\[n]\media" and then enter
mklink /d BetterImageGallery ..\BetterImageGallery. This will create a directory link at "\data\sites\[n]\media\BetterImageGallery" pointing to "\data\sites\[n]\BetterImageGallery". mojoPortal and IIS won't be any the wiser.