Until recently, if you used the drag-and-drop method to add images to messages in Gmail, it would insert them inline. Now, you can include them as attachments instead.
As Google Operating System points out, how you attach the image depends on where you drop it. If you place it in the body box, it will be added inline. Drop it at the bottom, above the text “Drop to attach”, and it will be included as a separate attachment. This fixes a fairly major annoyance that’s been around for months, and should be live for all users now. (Take note Apple: not everyone wants images embedded in email.)
Attach Images in Gmail Using Drag and Drop [Google Operating System]