Can I Change An Href In Javascript?

I am using some javascript code to change an image on an html page. I would
like the user to click on the image to bring them to another page. I need to
change an href to include an id value based on the image.

Here's the process:

1. select an image from a drop down list.
2. view the image on the same page.
3. click on the image to bring user to another page.

Here's my attempt at it:

function nextimg() {
txtString =
document.Topics.sel_val.options[document.Topics.sel_val.selectedIndex].value
document.image1.src = txtString.substring(txtString.lastIndexOf("&") + 1);
document.frmPic.href="details.asp?id=" & txtString;
}

I also think that txtString is not what I need. I believe it is a text
string (12345.jpg). So I will need to parse the string as well to get the
"12345".