I was wondering the same thing a couple of days. According to the docs:

• assets - a directory that keeps source code for your front-end assets, typically JavaScript and CSS. These sources are automatically bundled by the esbuild tool. Static files like images and fonts go in priv/static.
• priv - a directory that keeps all resources that are necessary in production but are not directly part of your source code. You typically keep database scripts, translation files, images, and more in here. Generated assets, created from files in the assets directory, are placed in priv/static/assets by default.

TL;DR: assets are for files that will be bundled by esbuild. priv/static is for static files. Hence, images go in priv/static.