Are the images and icons in the build? You can use the path as same as mentioned before.
For building build automatically, you can use a bash script. It could edit the manifest file with the brand name and select the desired folder to attach into the build. Then zip the file.
We use bash scripts for "customizing" builds for QA. We have different parameters to control the apps behavior (such as connecting to stage APIs, running with proxy, and so on). QA team uses a bash script for changing the app builds they receive, without needing to unzip, edit manifest file, and zip again.
I'm looking about assembleBrandDebug to a better understanding of what are you trying to do, but it seems similar to the ones that I'm telling you. If you need help with the script, I can give you and example π