SimpleViewer is a Flash image viewer and the images within the gallery are part of a Flash presentation and are isolated from any CSS that a theme may introduce.
The only way to have rounded corners on your SimpleViewer gallery images would be to:
(1) Purchase SimpleViewer-Pro from this web page.
(2) Upgrade WP-SimpleViewer from SimpleViewer-Standard (which it comes bundled with) to SimpleViewer-Pro by following the instructions here.
(3)Use the following SimpleViewer-Pro configuration options in the Pro Options text area of your gallery’s setting window.
imageFrameStyle="ROUNDED"
imageCornerRadius="10"
For reference, a list of all SimpleViewer-Pro configuration options can be found here.
