Reinstall the Mail app
You should be able to extract Mail.app from the OS X installer packages using Pacifist.

Re-installing Mail however is unlikely to solve your problem.

Mail has an option to scale images. When composing a message with an image attachment a pop-up appears in the header section to set the desired image size. Make sure that is set to “Actual Size”.

Next check if you have any Mail plugins installed that could cause the problem. Try removing plugins and adding them back one by one.

Next create a new account on your Mac and set up Mail there. If the problem does not appear, it is linked to settings in your account. Try resetting Mail preferences: trash the ~/Library/Preferences/com.apple.mail.plist file and restart you Mac. Mail will create a new preferences file. If that does not work, move the old file back in place and restart.