I need help with a script for my squarespace website

Hello, I am using the avenue template on squarespace. See my website for reference http://www.neiheiserargyros.com/work/

The work index has several images on a grid. Each image then takes you to a gallery with other images.On the index page, i'd like to change the image you see with another image when you place your mouse on it. Right now every image just becomes more transparent but i'd like a completely different image to show up within the grid. I need help with the script for making this work.

Thank you,
Xristina