Hi Dennis,
I may have this totally wrong, so please forgive me if I have. Also I use a Windows PC, not Apple, but the procedure should be the same.
I note you refer to "download". This normally refers to taking a photo from the database to your PC, only permitted for retrieving your own photos. Transferring from your PC to the database is "uploading".
Bring up the the aircraft's registration/tail number. There is only one aircraft profile with this number (left column). And no photos. Under the large letters of the tail number, it says, Have a photo of this aircraft? Share with others. Click on this link to bring up the upload page.
Click on Browse to select the file from your PC.
File and registration are now completed. Fill in the remaining fields marked with a red asterisk. As it is air to air, leave Airport where taken blank, put details in the Description box. Accept the upload conditions and click on Submit.
This should complete the upload. Again, apologies if you already know all this, but as no Apple user has replied, I decided to jump in.