Hi,
You don't give enough information to fully answer the question.
The "Mixed" application is not relevant to your issue unless you have made edits to the NAML code in your forum, nor should being in the "Members" group.
By default, all users, even those in the "Anyone" group are granted rights to "Create_Topic" and "Reply" and with those rights a user may insert an image.
I place a FAQ about image posting on all my forums. Take a look at a sample at:
http://seahawk-forum.968426.n3.nabble.com/Inserting-Images-in-the-Forum-tp1599211.htmlFeel free to copy the contents of this to your own forum.
Most likely they are posting a file direct from a camera. These days such files will break the size limit permitted for image uploads.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.