Size of image which can be worked by GD library on server depends on your server settings (memory, time, cpu, ...) because thumbnails are created by your server, then the settings needs to be checked there.
or you need to upload smaller images (in Phoca Gallery there is no limit, you can upload larger files, just set it in parameters, the limit is by the server because server makes the thumbnails. And creating thumbnails is time and memory consuming process, so mostly servers have problems with large images )Jan