Attached pictures not shown at recipient

I send a lot of mails to many different contacts. Sometimes with attached pictures like png and jpg files. Quite often I get people responding to me that they cannot see the attached file. And their response will almost always include the attached file, proving that they *did* get it. Often it is a Microsoft client, but I don't know for all different cases. Sometimes I can explain that they need to check the list of attached files, if the pictures do not show inline in the text area. But other times they insist that they cannot see any traces of the attached picture, whatsoever. This did not seem to be the case when I tested AirMail, for example. No issues with attaching for example PDF files. I remember something about a configuration setting for attaching inline vs. not inline. But I cannot seem to find it anymore? What can I do about this. Is it possible to make Mail.app attach files in a manner, that "all" mail clients are able to understand?