OS X Mail defaults to including all attachments at the end of emails. While useful for some file types (ZIP archives, programs, etc.), I still prefer that images be embedded exactly where I paste them in a message.
This insert-at-the-end behaviour can be disabled by unchecking an option in the Edit/Attachments menu:
Unfortunately, the option always reverts to being checked again for subsequent emails, with no obvious way to disable this behaviour permanently. As it turns out, the solution is simple:
If you disable this option while editing an email, it will remain disabled for that message only. If you disable this option from the main window, it will remain disabled for all future messages.
Tangentially, the option to “Send Windows-Friendly Attachments” can only be toggled when you are not editing an email. When you have a message screen open, the option is greyed out.
These options seem like perfect candidates for Mail’s Preferences menu. Why they exist solely as menu options and why they behave this way remains a mystery to me.