@AndyCOn a Sitegroud Shared host you cant increase the vars by you self, siteground has to change that. We are on the grow big plan and they set mine up to 9000 and we have no problem with generating 2000 pictures.
Look at your php.ini file
max_input_vars = 9000
zend_extension=/usr/local/php56/lib/php/extensions/ioncube.so
zend_extension=/usr/local/php56/lib/php/extensions/ZendOptimizer.so