I am not familiar with Github's specific case with regards to how it hosts your website, so I cannot answer for this specific case. However, for any data (whether that is images or those other files you were talking about) to be received by the client(s) (the viewers of your website) the files/data either needs to be published along with your html, css, and js (1), or you need to develop a backend system that can fetch this data from a server on which the data is stored (2).
Case (1) can be incorporated by simply publishing an entire folder which can house your html, css, js and have separate folders within itself for any data that you reference, such as images or other documents. The links to these data should also be adjusted in your html and js to ensure that they now point to the new locations of the data.
Case (2) can be incorporated in multiple ways, but I am not sure if Github hosting allows/runs a backend, which is what this case relies on. An example of how, in general, one may fetch resources from a server so that they may delivered to clients is here.
I have found a stack overflow post that says that Github does not process anything on the server-side (backend), however this is from 2018, so it is possible that things have changed. In the event that Github cannot host your site as per your requirements, note that there are various other free hosting services that you may find via a simple google search.