Custom HTML where to put it in a Child theme

This is the code I want to EDIT or CHANGE
div id=”mainphoto”></div

I have a child theme, and by Default the home page image is .PNG It is called main.PNG
I want to change it to be a .GIF image instead of a .PNG image.

I Want to change it in the child theme, so it does not get overwritten if there is an update to the Core theme which is The Dating theme.

I have obviously tried changing the Functions.php in the child theme to say main.gif instead of main.png but it does not work so perhaps im not putting the right code in the right place so that it is not overwritten.

I know png is better quality, however I have a very good reason for needing to change it to be main.gif ( I want an animated gif there NOT a slider !! ) Can anybody help with a suggestion please?
🙂