How/where can I get the little file upload icon along side the icons for bold, italic, URL link like you have here on the minibb support forum?
It is another add-on called "File Bank
Here you may find a solution
I've posted to work out the specific error messages when you're trying to post an empty message.
The specifics of posting an empty message is related to the core of miniBB, not this add-on. It will give you the same errors even if you don't have this add-on installed.
"You must enter some text with your image" text is already present ABOVE the upload Browse fields. For dumb users you may enter it in large red letters, it's up to you.