I'm having an issue with html roll over images in the header of my website. I'm using html code for the menu bar but the images are periodically disappearing in the test page and are missing when I upload to the web. One minute they are working the next they are not.

I'm not sure if it is a bug because it was working previous to this or does the code need some tweaking??  I'd appreciate some help on the matter.

Thank you in advance.