There are two ways to add Images to content: the Attached Images field and the Image Browser feature of the Body editor.
Attached Images
This is the easiest way to add images but not all content types have the Attached Images field. A limitation is that you can only add one image this way, and that you do not have control over how the image displays. Choose an Image to add, add a title (optional), click upload, and then save the content item.
Image Browser
The Image Browser will allow you to add images to any content type with a Body field. The best way to use the Image Browser, is to first upload your Image using the Create Content > Image form. You only need to add a Title and select the image to upload, the other fields are optional and will not be used in most cases. After creating the Image, go to the content item you want to display the image within, and click the Image Browser icon (