A common question that I receive is: How do I attach images in the Messages application. I’d have to agree that the method to insert an image into a text message it is not very obvious in the Mac version of the Messages application like it is in the iOS Messages app. In the Mac version of Messages, there is no menu item, no icon or no keyboard shortcut (that I’m aware of).
You can however, attach a picture to your text messages in Messages and there are a couple of different methods for doing so. The quickest way is to select the image in the Finder window and then drag it to the Messages entry box in the Messages app. The image will then be imported into Message and become part of the message.
An easier way perhaps is to open an image in an application such as Preview and use the Share feature. Click the Share icon on top of the window (in those applications that support this feature) and choose Messages from the list.
A small message box appears with the image inserted in the entry box. You can also access the Share feature from within a Finder window. Right-click the image you wish to share, point to Share from the contextual menu and click Messages.