Hopefully that issue will be fixed after SMF gets finished with the maintenance he is about to do.
There are probably several things that could cause that button to not show up. A few off the top of my head would be.
1. A browser issue. Browsers like FF often have problems displaying images properly. (clearing out cache usually corrects the problem)
2. If its a custom image your using (one you made for your stylesheet), the link to that image could be broken.
3. Adding to #2 - the image hosting site could be doing maintenance and when that happens, images that you are hosting often dont appear till they are done.
4. If its the default SMF button image, something could be wrong on that end and as I said above, hopefully it will rectify itself after the maintenance.
A few ?'s I would probably ask those members is, DId it just start happening? HAve you recently added any plugins to your browser (FF, opera, etc). Try to narrow it down.