I’ve found this to be a real annoyance, especially with a business where I have to send images in emails often. Mail.app by default displays images in-line, 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 behaviour 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
That will make every attachment you send act like an attachment instead of a pretty unusable decoration.
If you decide this isn’t what you’re looking for, to restore in-line attachment viewing type:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Restart Mail and you’re back to normal.