Thank you Frank, I found several images of portraits, but they all seem to have been used without any attribution, so I suspect they have been used illegally. While I could contact the owners of the blogs on which they are used and ask permission, I feel it would be a fruitless exercise if they themselves do not have permission to use them. There are no images available on Wikipedia or Wikimedia, and I have been unable to find any website associated with the family or the family home. There is an image of a portrait Mary's entry on Find a Grave, and I have sent a message to the contributor regarding its provenance and my potential re-use, but what I really need to find is where there is an original portrait for which I can request permissions.
When we upload images to Wikitree we are asked to
"Say where you got this image, e.g. if you took the photo, if it was scanned from a family collection, etc. This is required. If you found the image on another website explain why it is not copyrighted or how you obtained permission (images on Find A Grave and most other websites are copyrighted)."
I've tried the National Portrait Gallery and the Royal Albert Museum in Exeter... and really in asking this question I was hoping that someone might think of a potential repository I hadn't yet thought of!
Sally