If you installed Picsee on macOS, you have installed the extension. The only thing you have to do is to make sure you have enable Picsee extension in Safari Preferences and confige right. ......
Open Safari Preferences, in the Extensions tab, please check on Picsee - collect and download images extension.
Click Edit WebSites to config Picsee extension to run on websites you want.
One more thing, you must have 15.4 or higher version Safari to install this extension. After do all things, you can drag images on webpage to collect them in Picsee.