Having an issue with forum Preview function

If I have “show preview” enabled, and I add a picture or post code, the post button disappears off of the bottom of my screen. And I have no way to scroll down to click it. Using the scroll wheel on my mouse just scrolls the window above it. It scrolls though the replies in the thread. Clicking in my post box doesn’t do anything. If I delete the picture or edit out text it eventually comes back on screen and I can click it. If I turn preview off all is fine. I hope this is the right section for this.