Q What is the best way to copy images from Photos? I have tried looking inside its library in my Pictures folder, but that looks a real mess.
A Photos, like iPhoto and Aperture before it, stores its images in a labyrinth of folders, inside a library which poses as a single file. Whatever you do, you must not tamper with the contents of the library directly: it is a sophisticated database, and any attempt to fiddle with it will cause problems, and may break the whole library if you are unlucky.
Photos provides several ways of copying images out of its database. Probably the most direct is just to select one or more images in its browser, and drag and drop them to a location in the Finder.
If you want more control over resolution, format, and image compression, then select the images which you wish to copy out, and use the File menu command to Export them. You can set the popup menus to specify the exported images exactly as you wish, then where to save them. Photos will retain the original images in its library, so that you can manipulate your exported images as much as you want, without any fear of affecting those originals.