I assume that you're probably using PreTeXt through the CLI. If so, you will want to place your images in the "assets" directory. I typically create a sub-directory called "images" so images go in "assets/images".
Best practices are to include each image in a variety of formats (svg, pdf, etc) to facilitate conversions to HTML, print, and so forth, but that's not strictly necessary.
In your PreTeXt source, you will use an #image tag with the @source attribute pointing to the location of the image inside the assets directory. For instance, you could say something like
<image source="images/my_image.jpg"/>
assuming my_image.jpg is in the assets/images folder. If you have provided my_image.svg and my_image.pdf, you can say
<image source="images/my_image"/>
and PreTeXt will choose the appropriate image type for the conversion.