You have a few choices. First, click Select File and then in the dialog box that appears, click Pictures in the left sidebar under Media and select from the Photo Library.
You can also just have pictures open in another window and drag and drop from Pictures in this dialog.
You can usually also drag and drop from the Finder or Images to the "Select File" button on a site, but it depends on what they allow for that site.
You actually did a tutorial on this for a while back, and that's basically the same today: https://macmost.com/choosing-photos-to-upload.html Chapter19659005]