Yes, you are right.

The image hover is based on a VERY old bit of javascript code. I think I am right in thinking that most of the people who have worked on IH at various times think that the hover effect is horrible and so have not paid it much attention. Another solution would be to not use the image hover feature at all and just not load this javascript at all.

Or you could just ignore the 'error'.

With jQuery being promised as standard on a future version of Zen I guess it is likely that all this will get re-written at some point ...