If you have not already got it, get the app photo mosaica!
Open it. The top part shows how many photos will be used to create your mosaic, the higher the better quality! Click either choose an image or take a photo
After choosing your image, press start
Wait for a few seconds
This is your final picture! U can pay to get a real poster of it but can just take a screenshot and crop for a cheap version!!
Hope this helped!