As the name suggest, 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.
The image scale can also be chosen as a number between 100% and 500% from the Scale image dropdown field.
The higher this number is, the higher the resolution of the exported image will be. 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.