I don’t have much knowledge about all this but what helps me a lot is experimenting with things.
I would try the following things:
1. Create /image folder in child theme
2. Upload your edited “inf-scroll-loader.gif”
3. Check if that works
If not then continue with the following steps:
4. Create /js folder in your child theme
5. Upload the “graphene.js” file
6. Check if that works
If not then continue with the following steps:
7. Edit the original “graphene.js” file in your main theme (not child theme)
Find: “/images/inf-scroll-loader.gif”
Replace with the full link to that image!