The InsertImage instruction replaces a shape in your template with an image. You specify the name of the shape to replace and the URL of an image to insert in its place. An image can be either a .png or .jpeg file, which like .pptx assets must be accessible by the server at the link you provide. The image will keep its aspect ratio and be scaled to fit the placeholder.
Here's an example template with a rectangular placeholder where we'll insert a customer logo .png. The screenshot on the left shows the PowerPoint app open to the selection pane, which lets you see and edit the the names of all the shapes on the slides. In the tailoring instructions we specify the shape name, not the text within the placeholder box.
Run this shell command to insert the logo image into the template. It will download a logo-inserted.pptx file to your current directory.