At its simplest, dragging an image on top of the icon would be the most straight-forward method. Cropping mismatching aspect ratios could be a set default, as in 'cover' or 'fill'.
But that's still a hidden interaction, and if we're aiming for "Mac like", there needs to be a discoverable, menu based noun->verb interaction as the primary method. I propose making the 'Bookmarks' sidebar items selectable, and a menu item 'Edit' available in the 'Bookmarks' menu when applicable. The current 'Edit bookmark' dialog window gets an added image well for dropping/clicking on.
Right now, you can right click on the sidebar items, revealing options such as 'Rename' and 'Edit', but of course this contextual menu is also a secondary shortcut and doesn't quite make it. The primary method should be available in the 'Bookmarks' menu in the menu bar, making sure all actions can be achieved using just one mouse button.
(Edit: found the Bookmarks > Edit Bookmarks view, too; that covers the 'primary' case.)
TLDR; Select item in 'Bookmarks' sidebar and invoke 'Edit…' from 'Bookmarks' menu.