How do you make your own painting?

[Mackiiboy]

If you're going to use his method, be sure to enable static physics to the painting entity.

I just have one problem. My image is a bit transparent in Level Editor and in game, do ya have any idea why?

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EDIT:
I figured it out, I screwed up a thing in the material editor. Needed to tick another box.

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New Questions:
After closing and reopening level editor + save, the image disappears. Is there any smart method to fix this?
What about putting a new folder in the main-folder named something like "material" with my .mat file, and put a copy of that one in my custom story?
Will the game find that .mat-file if someone else will play my map in the future?