Hello

I read the first bit of the thread and a bit of the rest, it's all a bit tl;dr for me.

If you go to:
includes/functions.tpl.inc and find "<img" you'll find the html which is created for newly uploaded images, inline is the alt="[image]" you're after. Insert a blank space if you wish.

This will only work with newly uploaded images.

As suggested by Alfie

Tschö, Auge