Well, I was toying around in the forum and discovered a way to make it work. A bit of a little hack really.
Here is the format:
And here it is parsed below...
If you inspect the code of the page you will see the alt attribute is present. Do not add quotes and use dashes or underscores (although you MIGHT be able to get away with spaces in some cases). I can confirm this works on SMF BBcode, but have not tested elsewhere.