Not entirely free but cheap and non-scammy.
You do have to pay to get a high resolution version of your mosaic that would be suitable to print in a sizeable version, but you get to preview the finished product first and it's pretty easy to use. The fee differs per size, but the biggest highest rez is like 10 bucks.
I've used it for a personalized gift before.
Thanks, recommended AndreaMosaic, though and it's all I was looking for.
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