How do I locate specific part of code that affects ssl?

The gif is being loaded by your infinite-scroll plugin. The gif itself is in the uploads folder, which isn’t where it would be if it came with the plugin. This suggests that the plugin has settings where you can choose your own loading gif. Find the plugin settings and there should be a setting for the loading image. Then you just need to add the ‘s’.

If this is the plugin you’re using (it has the same folder name), I can see the setting in its 2nd screenshot: https://wordpress.org/plugins/infinite-scroll/