I’ve found this to be a real annoyance, especiallly with a business where I have to send images in emails often. 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
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 inline attachment viewing type:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Restart Mail and you’re back to normal.
One more thing, if you’re having the issue that Mail is resizing your image like Lissa describes in her comment below, after attaching a file, make sure that you select “Actual Size” from the “Image Size” pulldown in the lower right corner of your message window.