You could try the clone stamp tool in Photoshop. I've used it for removing dates that some digital cameras put in the corner of a photo, but it would be more difficult for a watermark such as shown on the example. The clone stamp tool takes a sample of another part of the image, and you use it to paint over the item you want to remove.
If all else fails, why not ask the museum if you could have an unmutilated copy, as I assume that you need it for a good reason m8.
I notice that all the pix from the museum website have got that stamped over them, but they will undoubtedly have the originals, and might let you have a copy.
Worth a go.