As the name suggests, Export Graph as Image will export a Maltego graph to an image format. Clicking the Export Graph as Image will open the following window:
The file type to can be chosen for the File of Type dropdown field. Image file types can be chosen from png, jpeg, bmp or html.
You can then choose the Image zoom level, the width and the bounds of the graph that you would export as an image. The width updates itself based on the zoom and vice-versa. Therefore, the higher this zoom level is, the higher the resolution of the exported image.
Keep in mind that with large graphs a high image scale can result in very large image files.
Once these options have been chosen, clicking the Save button will save a copy of your graph to the selected image format.