Hello,
Unfortunately currently GrapheneOS launcher can't import and utilize custom app icons.
Some apps can change their own icons natively.
I agree that this would be a nice feature to have!
That said, as GrapheneOS is focused on security and privacy improvements first and foremost, this may not be possible to implement at the moment.
GrapheneOS does aim to be highly usable as well as secure, so we thank you for pitching in with this idea!
Keep in mind that the project could also use more resources in order to be able to facilitate more requests like this.
Please consider donating. If everybody pitched in, the project would be able to have more full time developers working on all kinds of new features!
You can find out more information about donating to the project here:
https://grapheneos.org/donate
Thank you!