Mail.app displays images inline by default which can be a problem as most of the email clients out there won’t recognize them as attachments.
Of course, it is possible to right-click any image and select to view it as icon, but it gets annoying when you’re sending several images at once.
To turn off inline image display altogether, open Terminal (Applications>Utilities>Terminal), copy the code below and press Enter.
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
If you’re unsatisfied you can revert it by typing:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Restart Mail.app and that’s it!