You may not directly upload images to MFFA forum posts. Rather, you may upload to some 3rd-party image hosting web site and then copy and paste the image URL in your post input box field:
From RULES AND GUIDELINES:
Uploading / Sharing Files
For uploading files and sharing files with others on the forum we strongly recommend using one of these websites for each.
FOR IMAGES
http://photobucket.com
http://imageshack.us/
http://tinypic.com/
http://postimage.org/
http://imgur.com/
http://someimage.com/