I do recognize that problem too, but that's not the same as the situation I described above where I don't have an image. If I look at the html code in source view, there is no image, nor an image link.
Back to my original prolem:
I did have the exact same problem again once today. That was on my first forum visit of today. On all other occasions of new PM's, the notifiction showed as it should.
I also got message from another forum member who had the same experience. He was logged on and there was no indicator flashing.
As soon as I sent him a PM, he noticed the flashing envelope and went into his PM mailbox, only to discover that there was onother unread message waiting for him since yesterday.
There is one similarity in all occasions that I have recognized thus far, and that is that those non-indicated PM's were sent on day x, while the receiver was logged off. And where the receiver didn't log on until the next day.
I will keep watching for any predictable behaviour.