Mail.app by default displays images inline, and most email clients won’t recognize them as attachments. If you right click (or ctrl click with a one button mouse) on the image you can select to view the image as icon, which makes it behave like a normal attachment. To make this the default behavior you’ll need to use the Terminal to set the preference. Terminal is in
Applications Utilities. Open Terminal and type:
$ defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
If you decide this isn’t what you’re looking for, to restore inline attachment viewing type:
$ defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Restart Mail and you’re back to normal.