How to set decal image (C++)

nope. Doesnt work, says invalid attachment

I just set up the blueprint on a test level to download this image - http://cdn.bulbagarden.net/upload/thumb/0/0d/025Pikachu.png/250px-025Pikachu.png

This is my level start blueprint

Here is my material (very complex!)

And this is it in game with log messages and picture of an electric rodent.

So using the blueprint definitely works. Next step is figure out what you’ve done. Have you added log statements (like mine) to check your code is definitely firing?