How to export photos from old MacBook Pro to new one?

hinfinityl wrote: … the best I can do is screenshot the large previews and just use them as low resolution versions

There may be a better way. First, make a copy of the Photos Library, and move it out of the Pictures folder, just to be safe. Then open the Library Package by right-clicking (or control clicking) the icon to get this menu

and choosing Show Package Contents.

I don't really remember how High Sierra looked inside ( it changed in OSs after that,) but you should find some folders that you can look around in--maybe resources or renders or derivatives (the names have changed.) You should find the thumbnails, which are pretty useless, but you may find different higher resolution versions. Ignore files that are "plist" or other crazy kinds; you're hunting for jpgs. Looking in one of my Library Packages, I found some that are thousands of pixels, megabytes in size, that are way better than what I could get from a screenshot. If you see pictures in different groups of folders, choose the folder with the files largest in megabytes.

Messing around in the Library Package, especially changing files, can make the Library unreadable by Photos, so that's why is said to work with a copy. This is a little weird, but I created a new Library (close Photos and option click on the Photos icon,) and then imported my entire "Renders" folder into the new library so I could see them better.

I'm afraid this sounded more complicated than it is. The Package is just a protected folder. Look in there to find the best pictures, and use those. And do it with a copy.

I hope this gives you better options. Let me know what you decide!

You could try to drag that photo to Terminal window which then displays something like:

/Users/matti/Pictures/Photos\ Library.photoslibrary/originals/0/0F78CF8A-729D-4BC7-91E6-2C89EDD306A1.heic

Warning: do not in any way modify the library's contents because it might corrupt it! If in doubt make an extra backup of the library and use it.

Then drill to that folder and see if the original photo is there.

If it is there, copy it to another folder outside the library.

hinfinityl wrote: … I’m under the assumption they exist somewhere.

That's not really true. You say you can zoom in, but can you Edit these pictures?

Photos has thumbnails and previews that don't have the full resolution of the original, and you may be looking at one of those rather than the full versions. The full picture actually doesn't exist anywhere-- it's created from the original and the database's recording of any edits or comments you may have made. No file of that version exists until it's created for export. Photos concentrates on images, not files, and so you can't treat it like Finder.

Photos can lose touch with the original if the database or the Library package gets corrupted in some way-- by opening the package and messing with its innards, as Matti Haveri warned against. Also for instance, it's possible (and totally to be avoided) to set Photos to keep the originals in Finder folders rather than importing them into the package (called a Referenced Library.) If that was ever done, then your original pictures may be on the computer somewhere, or they might have been deleted by now.

You can settle for the Preview versions if they exist. You may be able to drag a picture from Photos to the desktop, getting the preview version, or maybe not. If you go digging into the Library Package, (and then you really really need to have a backup copy,) then you may find thumbnails and preview versions, though that may involve using QuickLook for each one. High Sierra stored pictures differently than newer OSs, and I'm not sure exactly where the previews were kept-- if they were.

Let us know how it goes…

Oh-- I forgot to ask-- can you do File>Export>Export Unmodified Originals?