Hi Oalias.
You are certainly on the right track. The current Tree 18 objects allow you to use a custom picture for plan and 3D views. ArchiCAD already uses images so I don't see a problem, just don't make them high resolution.
As for the file format, judging from past experiences with imaging, PNG should be the correct one to use as it allows for transparency setting instead of WHITE background.
I haven't used this either but as an ArchiCAD subscriber in Australia you can download an OZBREED library from your support.graphisoft.com.au login. I believe it has a massive amount of tree images you can use.
ArchiCAD 10 - 25 | Windows 10
ARCHIcreate | Perth, Western Australia
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