[TYPO3-english] preview for own FCEs?

[Bernd Wilke]

I already asked in typo3.slack#cig-ext-frontend, but got no answer.

I created some own FCE. my first approach was based on an old data 
structure and I stored images in the field `image`,
now I want it more like 7 and changed the field for images to `assets`.

All works well - aside the BE-preview (page-module) of the CEs has lost 
the  preview image. What is needed to show the preview image again? (the 
images where shown with the field `image` automagically)

or asking more generally:
is there a documentation how to create a BE-preview (for page module) 
for my own CEs?


bernd