Replace:
if ($img_size = getimagesize($_avatar)) $_avatar .= agoraIntegrationJomsocialAddonHelper::getDimensions($params, $img_size);[/code]

with
if ($img_size = getimagesize(str_replace(rtrim(JURI::root(),'/'), rtrim(JPATH_ROOT,'/'), $_avatar))) $_avatar .= agoraIntegrationJomsocialAddonHelper::getDimensions($params, $img_size);