Trouble uploading images from Photo's to other sites, e.g. Pinterest.

If you're using this window in your browser then create an album and place it near the top of My Albums section in the sidebar so it easy to find the photo you want. Or you can create a smart album with this criteria (changing the number to fit your needs):

It would look something like this:

I believe what you're experiencing is that you have the iCloud Library enabled with Optimized Mac Storage. So when you click on an image to use it the full sized version has to be downloaded before you and use it in any project, edit it, print it or use it outside of the library.