This feature is only available in the desktop editor.
Normally individual images within an image content block cannot be selected. This makes annotating the images easier. This article will show you how to select images if you need to.
In order to select an image you must use the Image tool. When this tool is active you can select images and move them around. In addition, you can change the border type of the image.
Alternatively you can right-click on an image to select it. This will select the image and bring up the contextual menu for the image.