iMessage will not deliver pictures... even though 'Delivered' receipt shows

Hi everybody!

I currently use iMessage on my iPad, iPhone and iMac and up until today I'm having issues with sending pictures from my iPhone.

I could send and receive pictures from my iPad and iMac to my contacts, but I am only able to receive pictures from my contacts on my iPhone. If I attempt to send a picture through iMessage to any of them, the receipt will be labeled as 'Delivered' but they wouldn't receive the picture.

Interestingly enough, on my iMac's file transfer list in the Messages app, the progress bar is indicating that the pictures are ready to be transferred, but obviously nothing shows up and it is stuck.

I've tried everything I could possibly do: reset PRAM on the iMac, turn off iMessage on all devices; reset devices and turn back on, unlink iMessage from my iPhone, etc.

Any ideas? 😟