I cannot upload photos to any websites

Can anyone help please??

I can't upload any photos to any websites. I either get the message "Open photos to see your photos on this list" or "loading images" which then does nothing (I've left it for a LONG time)

I have checked the following -

Photo library is definitely listed as system photo library (it's greyed out)

All updates are in place

Photo library has been backed up and deleted, have started new library with just 71 photos

I am running on Mojave

I've now spent nearly 2 hours on two different chats with Apple Support, have tried running in safe mode which has not solved the problem. Their solution now is for me to upgrade to Big Sur but I have nowhere near the required available memory, so it will be an enormous undertaking, just to be able to pop a photo or two on Facebook!

Does anyone have an alternative solution??? I just can't believe that it's an issue that can only be resolved by wiping half of my data!!

Would really appreciate some help - I'm not particularly computer literate though, so please be patient.

Many thanks