When I'm composing a mail in Gmail I want to insert an image directly in the mail's body, but there is no "Insert image" button at the top. How can I configure it to be visible?
There are two ways to insert an image in the body of the mail:
To enable "Insert image":
Indeed, by default there is no button:
If you want to insert an image from your computer to the body of your email, use drag-and-drop from your desktop (or file manager) to your web browser.
This drag-and-drop uses HTML5 so it will not work on old browsers.
Alternatively, you can enable the button as described by Alex, but it is a Labs feature, so it "may change, break or disappear at any time".