mihomes - 11:21 pm on May 8, 2012 (gmt 0)
I read the original question too fast and missed the "image has text on the button" part. So I thought you were describing Mystery Meat Navigation and was all set to deliver some very emphatic opinions ;)
Backtracking: If the anchor text isn't visible, why is it there at all? Wouldn't the "alt" attribute of the image serve the same purpose? In fact it had better do so, since that's what your image-less users would see.
In the manner I currently use there is no image tag, therefore, you cannot use an alt attribute. Maybe I didn't explain it well, but the rollover button is created with css on an a tag as mentioned in my OP - there is no img tag used at all.