Thanks @Sinval, I'm glad you like xaringanExtra! Sorry for the delay in getting back to you... The answer is first to place the image file in the same folder as your slides, or in a subdirectory of the folder containing your slides. The second step is to use a relative path to specify the location of the image instead of a full path. here::here() always returns a full path, so this is one place here() isn't a good idea.
Here's what your directory might look like.
slides/
|- my-awesome-presentation.Rmd
|- my-awesome-presentation.html
|- fancy-figure.png
Inside your slides, reference fancy-figure.png like this:

Or with use_logo()
```{r use-logo, echo=FALSE}
xaringanExtra::use_logo("fancy-figure.png")
```