Create a separate entity that inherits from the system image entity. Then associate this entity to your entity for machine info.
If you must store the content you could use a Binary Type or Unlimited String and convert the image to a Base64 string.
However, an association between your entity and system.image as Austin suggested is the preferred method.
As a side note, saving images as true files, they are stored on the application node; saving image content as attribute of an entity will store this in the database. Something to consider, based on your specific case.
If your entity for Machine only ever needs one picture, then you can have your Machine entity inherit straight from the `System.Image` entity. This inheritance will give your Machine entity all the fields that the `System.Image` entity uses to store data about an image. If you need to storage more than one image, then you should follow Austin's advice above.