Add a Custom Sidebar Image Divider

possibly by editing one or more of your theme’s templates (?)

please post more details, such as the name of your theme, and a link to your site.

I am currently using the Prose theme and my site is currently not online but in set-up mode. I am looking for some CSS code to replace the default bottom divider with a custom image. For instance, in my current blog I use a row of flowers and stones as my bottom divider. I want do so something similar on the WordPress blog I am developing.

‘prose’ seems to be theme by studiopress;
please contact them for support with your question, as commercial themes are not supported by this forum.

general, what you seem to have now is a graphical element below the widget titles;
use a browser inspection tool such as Firebug to find out which css styles are responsible for formatting the widget title in your new theme, and add the graphic as a background image to the widget title.
pretty much the same way as it is now in your current blog link.