hai controllato i requisiti per utilizzare simple image gallery?
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).
..parli del template brander
per escludere che il problema sia il template
hai provato a impostare quello di default di joomla
e verificare se il problema si presenta lo stesso?