All image settings you can find in settings/image.ini. File that contains comments, so you should quickly figure out how to add new image aliases and so on. Don't change default file, use global override or per siteaccess. Create new file image.ini.append.php under settings/override or settings/siteaccess/(your_site_access)
If your content class attribute is image datatype then you can display it using
{attribute_view_gui attribute=$node.object.data_map.image image_class=large} where image class equals to aliases in your override image.ini.append.php
See also template design/standard/templates/content/datatype/view/ezimage.tpl for list all available parameters.