You can include an image via the <image> tag. You can use the @source attribute to provide a filename, likely prefixed by a relative path, that points at an image file. It is your responsibility to locate that file properly relative to your output, and that the file format is compatible. So, for example, suppose your source contained <image source="images/butterflies.jpg"/>. Then you would want to have a directory named images below wherever you process your output, or wherever you place your HTML output on a web server. The @width attribute can be used to control the size of the image. Widths are expressed as a percentage of the available width, such as width="60%". Instead of a width, you can also specify margins and the width will be deduced.
You may want to wrap your image in a <figure> to have it centered, and to have some vertical separation above and below. A <figure> must also have a <caption>, and the figure will be numbered.
You can also place an “anonymous” image (no caption, no number) almost anywhere you might place a paragraph (but not within a paragraph). Note also that the <sidebyside> tag provides some very flexible options for placing several images (Section 3.18) together, or combining figures with subcaptions.
If you wish to construct technical diagrams, with editable source, and perhaps including the use of macros, PreTeXt provides support for authoring with graphics languages such as Asymptote, TikZ, PGF, PSTricks, and xy-pic in addition to using Sage code to describe a plot or image. In most cases output can be obtained as smoothly-scalable SVG images, in addition to other formats like PDF or PNG. Making all this happen is one of the more technical aspects of PreTeXt, so read the details in Section 4.13 along with frequent references there to the pretext script described in Chapter 45.
For accessibility, every <image> should either have a <description> child, or it should explicitly declare itself to be a decorative image setting @decorative to the value yes. A description should minimally describe the important information in the image that is not already present in the surrounding text. Some screen readers may cut off reading a full descritption after the first 125 characters. For a complex technical diagram or an image from which the reader is expected to extract important information on their own, it can be challenging to write a description with the 125-character limit. It may be advisable to seek advice from an expert.