...Interestingly, I always received the user's pics when they initiate a contact but in the two above cases, no pic was attached...
That would be because you have "You must have a picture to contact this user" on your profile, so anyone that sends you a message and has a picture, means it will be automatically attached to the message.
...Is that a glitch in the system or is there any other explanation?..
I have had it happen to me a few times. In a couple of cases they where people that had contacted me in the past and changed their profile name, but a few that where the same scenario as you described.
So I would say "other" explanation would be more actuate.
Just like someone can post fake pictures, there are ways to get around word filters.