Hi @MrNappa ,
Do you want to use the internal icon's image to update image field?
I'm afraid it's not supported in PowerApps currently.
Because the icon property is an input property and could only be used to set the icon itself.
If you use this property outside, it will be invalid.
For example, I set an image control's Image to Icon.Add, it display blank.

I suggest you express your views about using internal images on PowerApps Ideas Forum. Your precious feedback will be very helpful for our work.
https://powerusers.microsoft.com/t5/PowerApps-Ideas/idb-p/PowerAppsIdeas
As an alternative way, I suggest you upload some local images to use.
In media pane, choose browse , select the default images that you want.

Then, you could use these images when the entity image is blank.
For example, your uploaded image named addimage, try this formula:
If ( IsBlank( item.'Entity Image' ), addimage, item.'Entity Image' )
Best regards,