I work in Service Department. As a regular part of my job, I put images in emails. I have to show people how to do things, and putting them as attachments and not inline would not be helpful.

At my old job, as an engineer, I also put images inline. I got emails all the time, or calls, asking why is this? The easiest way to show someone is in an email, with a screenshot.

Its not any different than an article in a newspaper or on a webpage with an inline image. It actually just works.