Image in UI Widget button don't get packaged

I’m creating buttons in an UI widget and assign textures to them on runtime-creation. The textures don’t get packaged.
I added their folders under Projekt settings → Packaging → “Additional Asset Directories to Cook” but they still don’t get packaged. On runtime the packaged app displays random other textures , looks like an memory/object misread from something else.
The additional folder to cook seam to be ignored. I managed to get them cooked by creating materials with them and putting them into my main level on hidden planes so they are included by the level itself but this is ridiculous.
How to do it right?