It means that you will have to either delete some of your pictures/attachments in order to post more...you are only allowed 50mb if you are using the forum site....if you go to your attachement management section in your user profile you can delete photos/attachements...but if you do the delete of pics then any pics you had in a particular thread or post will disappear....not the post or thread but rather just the pic/attach...
Some folks you photobucket and other photo sites and then attach that way....
I have never had this issue, I use tinypic.com. I was not aware of any quota limits on TFO. What ascott said makes sense, if you use tinypic there must not be any limitations. That would be a bummer if there were limitations.