Need help with image template in WordPress

Hi. I need to know how to modify my custom made image upload function in my wordpress dash.

When writing a post, I have an ‘image’ button, which lets me specify the file name which i would like to show in post, and it inserts a script.

I’ve attached 2 pics to let u see what I mean.

– step 1
– step 2

I need to change the inserted script slightly, and need to know which PHP file it can be found in. I have looked everywhere!

Thanks.