Choose 2 images to build a Rollover effect on the image tag that has id="img1" NEED HELP

Choose 2 images to build a Rollover effect on the image tag that has id="img1" NEED HELP

1.       Choose 2 images to build a Rollover effect on the image tag that has id="img1". Using JavaScript complete the following tasks;


here's a link to my entire website  http://jsfiddle.net/dL012dfu/