Mitch told correct things, you get much many advantages with the dedicated plan than with any other hosting package. Looking at the visitor numbers you provided, I bet that there will be issues with hosting the pictures part of the site on the shared plan because it can attract hundreds of concurrent connections from your forums visitors. However, situations, when one site or account starts utilizing a great part of server resources which include not only the bandwidth and disk space are not allowed on the shared servers.
Unfortunately, as I mentioned these forums are just a community of people having a relationship with Lunarpages and we can't have an idea about upcoming plan changes (or can't mention them until official announce is made). You need to contact the company management via the feedback address give your opinion and get the official answer.
As a temporary solution, I would recommend purchasing a second dedicated server and splitting your forums in 2 parts to utilize the combined bandwidth.
Please advise if you have any additional questions or concerns.