There is no "asset UI" as part of adventuron at the moment (there will be in future).
So, the best technique for updating multiple graphics at the same time is to zip up your PNG files into a single zip file, and "import" that zip file. It will overwrite the ids of the graphics in the assets / graphics section automatically. Alternatively, you can just re-import each image, and it will overwrite that single image.
The easiest approach is to name your png files the same id as the location they appear (if they are location images). That way each location doesn't need a graphics = "image_id" field.
e.g.
C:/somefolder/yourgraphicsfolderon
- hut.png
- forest_1.png
- forest_2.png
If you import those files then they will be imported with graphic ids "hut", "forest_1", and "forest_2". If you have locations called "hut", "forest_1" and "forest_2" then the graphics will automatically be applied to those locations, therefore updating the graphics is just a case of re-importing.