Rearrange single pictures in Photos?
The default views (Moments, Collections, Years), as well as all automatic albums (All Photos, Selfies, Videos…) are always sorted with the most recent at the bottom. This ordering cannot be changed.

However you can create Albums, throw your pictures in there, then rearrange them to your heart’s content.

To view your albums, press ⌘ cmd 4. You can also show the sidebar by pressing ⌘ cmd ⌥ opt s (it will be shown by default in macOS 10.13 High Sierra). Either option will give you access to your albums. Once you open an album, simply drag-and-drop it wherever you like with the mouse.

This will not affect the default views and automatic albums.