This is an excerpt from the documentation page of Simple Image Gallery at http://www.joomlaworks.net/content/view/79/41/
In order for Simple Image Gallery to function properly, please make sure that your server has PHP's GD image library installed (common webhosts have GD installed by default). Additionally, your site must meet all requirements as posed by either Joomla! 1.5 or 1.6+.
You also have to make sure that Joomla's /cache folder is writable, in other words, check that the permissions for this folder are 755 (or 777 in some rare cases)."
For additional help you need to consult your hosting company.