Instructions accept Markdown, so an image goes in with the standard Markdown image syntax pointing at the image's URL. Host the image somewhere reachable and reference it by that URL.
Video: Adding images to your survey instructions on YouTube
Some questions are far easier to show than to describe. A photo of the correct meter reading position, or of what counts as a functional handpump, removes a whole category of enumerator error along with the retraining that follows it. It costs one line in the instructions.