So I'm setting up a ancestor altar and I want to put a picture of some of my grandparents and great grandparents but my mum told me it may not be a good idea because they were hard core Christians that, from what I've been told, had a strong dislike for witchcraft and stuff. I can't decide if it would be disrepectfult to leave them out or put them in.
Along with what Neko said. Altars are not just for witch craft purposes. They can be created for remembrance, for example, the Day of the Dead altars have nothing to do with witchcraft. They are just beautiful structures created for a passed loved one.
Remembering your dead relatives is not a sin nor is it disrespectful. Just look at the Day of the Dead in Mexico. It's somewhat a religious holiday in the catholic church there. They set up altars with skulls and very colorful flowers. They visit their dead relatives in the cemetery and you spend the whole day there.