What's it called???....

I have an idea and I'm trying to find an image as a reference but can't for the life of me work out what it's called to find a sample.

Perhaps somebody who's reading can help...

I'm thinking of a pixellated/pointillism idea, where the image is made up of dots or squares of colour. I've seen pictures (sometimes protraits, but other images as well) that are made up of thousands of smaller photos of the same theme/person/image arranged in such a way that when you stand back you see the larger image almost perfectly. It's kinda like an optical illusion.

I'm looking for a link that might show this - does anyone have any ideas?...