You can include images when you start a new prototype with images or afterwards, when you set an image in your prototype.
Starting a new prototype with images
In this dialog, you can add as many images as you want. Then, you can choose to include them in your prototype. This option will be applied to all your images.
Including an image
When you set your image in your prototype, a new dialog will appear to ask you how do you want to set the image.
When you have an image in your prototype, you can do multiple actions over it.
If you show the context menu from an image component, you can find some image-only commands.
Those commands are also on the image toolbar and the image menu bar.
Here is a screenshot of the context menu of an image:
