selecting photos for facebook or eBay

Your screenshot is showing, that the Photos.app is not running. Launch Photos, so it can run in the background and create thumbnails for the Media Browser. If Photos were running, we would be seeing a tiny dot below the icon in the Dock.

Log into another vanilla admin user account, Set up users, guests, and groups on Mac,  and see if you can upload from there. This tells us if the problem is limited to your user account or is system wide.

Create a new, test library, make it the System Photo Library, sign into the Same Apple iCloud account as your original user account and try uploading. If you're able to there's something in your original user account that's the culprit.

If the library is large and Photos is syncing with iCloud Photos and "Optimise Mac Storage" is enabled it may take a very long time for the albums to appear. On my old Mac with a small SSD I have to wait occasionally for fifteen minutes or so, until finally some albums with photos will appear in the Media Browser where it is showing "Loading Images". How long have you waited for the "loading Images" to finish?

How much free storage do you have on your Mac? Have you tried to view the photos enlarged in Photos before you want to upload them to Facebook? Viewing them enlarged will force a download to the Mac, so the Media Browser can show them.

Hi

Look in photos preferences. Is the "use as system library" button greyed out as in the screenshot? If not, then click it, then try again.

As a workaround you may have to export the photos to a folder on the Desktop and access them from there.

I will add another interesting find: Open Safari, then go to file - open file: this time it will show photos!

But not when picking an image in websites that allow image/file uploading.

so it's clearly a bug. I have filed bug FB9075157 in Apple Feedback Assistant, I suggest everyone do the same especially if you can confirm that File - Open works in Safari and Preview, but Safari cannot see iCloud Photo Library when attempting to upload to a site such as OP mentioned or for example imgur dot com

>Did you manage to get it sorted if so please tell me how.

In the end I gave up on Safari and switched to Mozilla Firefox. It has the same issue, but I was able to codesign it and add entitlements. I tried adding the entitlements to Safari but since it's part of the OS it wouldn't let me do it.

I am sorry due to lack of time cannot explain step by step, but if you follow this guide you should be able to successfully code sign Firefox and be able to use it to select photos from iCloud Photo Library:

1. dump current entitlements for Firefox to an xml file https://apple.stackexchange.com/questions/52675/how-do-i-find-out-what-entitlements-an-app-has/52680#52680
2. edit the xml to add iCloud photo library entitlements com.apple.security.assets.pictures.read-write | Apple Developer Documentation https://forum.xojo.com/t/mojave-privacy-with-drag-drop-and-helper-apps/45677/8

    <key>com.apple.security.assets.pictures.read-write</key><true/>

    <key>com.apple.security.personal-information.photos-library</key><true/>

3.create a self signed certificate and code sign Firefox app with the edited xml. https://stackoverflow.com/questions/27474751/how-can-i-codesign-an-app-without-being-in-the-mac-developer-program/27474942#27474942

for some reason this didn't work for safari, it refused to codesign. I also tried to fix this for Signal for Desktop for Mac but again it failed on me even if it allowed codesigning. at least I'm happy it works for Firefox.

you would have to repeat this all over again (minus creating the certificate) after each Firefox app update... 🤦🏻‍♀️

...or you could use Microsoft Edge for Mac, which I observe it has all the necessary entitlements (but please do use Mozilla, it's the last line of resistance against giants monopoly in web standards)

>If the library is large and Photos is syncing with iCloud Photos and "Optimise Mac Storage" is enabled it may take a very long time for the albums to appear.

op said optimise Mac storage is disabled. I also have full originals, not optimised.

>On my old Mac with a small SSD I have to wait occasionally for fifteen minutes or so, until finally some albums with photos will appear in the Media Browser where it is showing "Loading Images". How long have you waited for the "loading Images" to finish?

it is completely irrelevant how long you wait, since there is a difference in UI between slow loading and not loading at all. when you wait to load, you see a spinner. in the op case (which happens to me as well) you see no spinner, activity monitor shows no cpu usage, fs_usage(1) terminal command shows no library file activity. this is a bug introduced in Mojave to make sure they kill our experience on Mojave, the last version that allows Collections and Moments which apple destroyed since Catalina and iOS 13.

>How much free storage do you have on your Mac?

sufficient:

$ df -h /
Filesystem     Size   Used  Avail Capacity iused               ifree %iused  Mounted on
/dev/disk1s1  116Gi   68Gi   43Gi    62%  878379 9223372036853897428    0%   /

>Have you tried to view the photos enlarged in Photos before you want to upload them to Facebook? Viewing them enlarged will force a download to the Mac, so the Media Browser can show them.

this only applies to optimised Mac storage. there is nothing to "force" when your full system library is stored on the mac

PS: the "Preview" Mac native app is able to instantly browse iCloud library photos. open it, then file - open and selecting photos would instantly display iCloud photos structure.

Post a screenshot of what you're seeing.

Do you have iCloud Library enabled? If so is "Optimize Mac Storage" also enabled?

Are you running a "managed" or "referenced" library?

If grayed out it already is designated as the "System Photo Library".

Are you trying to get to the photos in the library list the Media section in the sidebar of your browser as shown in this screenshot?