You can add "hideaddbutton=true" to remove the add button in photo galleries in source edit mode, and I use this tweak on my own wiki. Here's an example:
<gallery (all other gallery settings go here) hideaddbutton=true>
Ok Wigglepidea People .... I need your Help! I found something on Facebook that seid the Wiggles are Comming Back to Sprout ! can you find out for me ? I put the Post on my Timeline on Public so you can see it ..... I hope iit's true !!!