Bluetooth file transfer between Mac and android phone
If you have done everything in that HowToGeek article correctly, I don't see how it's not working. So maybe go through it step by step and check that you didn't miss anything.

Next thing may sound stupid, but have you tried turning both devices/Bluetooth on those devices off and on again? A good old reboot may be enough to fix this problem.

If both of my above solutions don't work, then I cannot help you further, however, I can give you an alternative (although that isn't what you really wanted). I use a messenger called Telegram and send images (sometimes files) from my Phone to my Laptop and vice-versa. Good thing is that there is a client for almost any platform which means you can access your messages (and in this case images) on any device you want.