Maybe it's more the hosting site that the pics are at that is the issue. Sites that require you to sign into might consider your pics private and not let anyone else see them. If that site has a "share" function, use the URL that it can generate to put pics into your posts here. But even with that, some pic hosting sites just don't work well with BF and other forums. So in that instance, use another hosting site.
But if you are specifically asking about sending pics via PM's, private messaging, I have not tried for several years. But I used the same linking tools that are available today for public messages.
If nothing else just send the URL to get to the pics. Unless the SPAM filters are different for PM's, you have plenty of posts to attach pics and links in your messages.