Perhaps there is a clash between the CSS of your SimpleViewer gallery and that of your current WordPress theme or an installed plugin.
You could test this theory by temporarily reverting to the default WordPress theme and deactivating all plugins (expect WP-SimpleViewer if you are using this to create your gallery). View your gallery after doing this to see if it displays as expected. If it does, then re-enable each of your plugins one-by-one (and check your gallery each time) until it fails to display correctly. The last enabled plugin will be the one that is causing the conflict. Do likewise with your theme.
You may also need to clear your browser's cache after re-enabling each plugin before reloading your gallery.
If you are not using WP-SimpleViewer, one way to isolate the gallery (to avoid any CSS clashes) would be to embed it into your post using an <iframe> rather than using embedding code with a baseURL. Please see Option #2 here for details.
If you continue to experience difficulties,please post the URL of your gallery sop that I can take a look.
SimpleViewer Support Team