I know how to do it with phpBB, but I'm not sure with invision. See if you can edit the template and you see index_body.tpl Probably not, but that's how with phpBB.
In the Admin CP go to "Images, and find the "No new post marker" and swith just the url of the images
EDIT
NVM I would say use invisionfree, pretty much the same thing but easier, i never use InvisionPlus