What is the best way to test if a file's name is anywhere inside of project's files

I am a web developer and my boss has just asked me to clean the images that are on our server. There are about a thousand files and I need to check which ones are used and remove the unused ones.

According to you, what is the best way to do this? My first idea was to make a script which would parse all the project files to check if any of them contains the image file's name. But there are hundreds of files inside of our project folder and this is the first time I am thinking about that kind of script. Wouldn't this method cost me a lot of time?

Do you have any other idea? Thanks a lot!