Google has added a new feature for Gmail that lets you attach any image that has been automatically backup to Google+ from your mobile devices. This option would be enabled using a new Insert Photo button in Gmail that lets you access all your images. Gmail already has an option to attach images from your Google Drive.
You can also resize in-line images after attaching them, just by dragging on a corner. Google+ for Android got Auto Backup option long back and the and the iOS app got full resolution image backup last year that lets you store full resolution images easily. This feature would be more useful to people who would wish to mail the images they shot on their mobile devices.
You have to enable the Auto Backup feature in Google Plus for the Android and iOS first. Since the Google+ apps for Android and iOS have option to backup images only when you are connected to WiFi, this would save your data. This feature would start rolling out today in Gmail on the web.