I fourth (?) the Nixplay suggestion. We got one for DH's parents and one for his great-grandpa. Each of DH's siblings had access and would go on every so often and upload some pictures of their family. It was super easy.
Google home hub is what we use for a photo frame. Has the added features of connecting for questions and stuff, but that may not be what you want. But you can send photos to it (by adding pics to specific google folder).
Another vote for nixplay. All they have to do is turn it on. Or leave it on, it has a motion detector. I can send pictures and I can invite anyone else I want to send pictures. That way she can't complain about not having pictures of the grandkids.