Twenty Twelve Gallery

I have a child theme. I know how to upload and add media. I can’t seem to create a gallery! I followed these directions: https://codex.www.ads-software.com/The_WordPress_Gallery – and it didn’t help…and I received this message: Fatal error: Allowed memory size of 94371840 bytes exhausted (tried to allocate 523800 bytes) in /home/studiosahl/wendywainwright.com/wp-includes/query.php on line 1577 – so, o.k. my images are too big? What is the largest size image that any given image can be?