Q: How much PHP memory do I need for the gallery thumbnail image creation?

A: That depends - read on to find out why

To create thumbnail images, the gallery first needs to read the original image into the RAM. Now you could say: my JPG image file is FullHD and uses 200KB , BUT....

the file you stored on your disk is compressed.In the computers RAM, the image gets uncompressed, so in the memory it uses about 8MB.
Now the thumbnail image needs to be created from the full image which also uses some memory.
Additionally, the whole Joomla system also needs some memory.

All in all, your web package should be configured for a PHP memory of at least 64MB, which should be the absolute minimum nowadays.
The default memory limit in PHP5.5 is 128MB - with this value, you should be able to create thumbnails from even much larger images.

If you ever get errors saying that the memory limit has been reached, raise the PHP memory limit (or ask your provider to do so) and try again.