If you right-click in your project Explorer. You can go to Add other > Image collection. Here you can add images, which you can use on your page (e.g. with the Image widget).
Another option would be to add an images directory in your theme folder (in your project directory) and upload images here. These images can be used in your css -> url(/images/image.png)
You can use the 'Image’ widget. And upload the image you want in the modeler.
When placing the widget on the page you are prompted to select the image from your project.
To upload an image to the modeler:
Then put the 'Image’ widget on your desired page and select the image you just uploaded.