Bear in mind that users may mouseover the img accidentally and may not understand what's happened when they're suddenly taken to a new page.
" onmouseover="window.location.href='newpage.html';" />
i.e. use with extreme caution (imho, don't use at all)
Also, bear in mind that those who use keyboard to navigate around pages almost certainly won't be catered for by this process.
You might partly account for more users by keeping the img within an anchor which has the new url as its href.
<a href="newpage.html"><img src="
" onmouseover="window.location.href='newpage.html';" /></a>