Help with Alt images

[totallybike]

Hi all,

I am having a problem with my products. I want to add a series of buttons along the top of the product description (see http://www.totallybike.com/913-reflex-psycho-jump-pink-2011.html). I have made all of the image files (the grey ones you see on the page above and a second set I want them to change to when the mouse is over them).

I have done the following:
In the description box in BO I have added the main image (including image description and title)
In the Advanced tab I have selected the tick box for alternative image and have then included the image source for mouse over and for mouse out.

It seems to work in BO (image changes when mouseover and reverts back when mouse is moved out) but when I save it comes up with the following error along the top of the page:

1 error

the field description (English (English)) is invalid

How do I get this to work as I am currently updating my entire catalogue and want to add this to all of my products!

Thanks in advance, Gary