This doesn't come up that often, but sometimes when I receive email with some kinds of large image files included inline with the body text of the email, Gmail presents the image in actual size, rather than scaling it to fit on-screen. When the image is very large, the image can display at several times the width of the region of the Gmail window for viewing message. Worst of all, Gmail flows the text of the message to the width of the image, requiring the user to scroll left and right to read the message.
This free Chrome extension fixes that behavior, configuring Chrome to automatically scale images to fit in the current message pane (as Gmail should have been written in the first place. Gmail's built-in image viewer is still accessible by double-clicking the image, so the user still has access to the full resolution of the inline images.
Gmail Inline Image Fit