What device are you trying to transfer to? Is the problem that the .png file is visible but the transfer fails, or that the .png file doesn't even show up to begin with?
I Bluetooth transfer files all the time from Pixel 7 to iMac. .png file are screenshots, and transfer fails with only .png files using android. Although, it seems at time the transfer does work, mostly not. These are some of the files.
Are you initiating the transfer using the phone or the Mac? (Full disclosure, I never transfer files via Bluetooth, so I'm not an expert at this -- just seeing if we can get more info first.)