Troubleshooting with <img> tag

I have a few <img> tags used in my theme, but they’re not show at anytime. One of them is “wp-content/themes/mix/images/rss_a.png”. What should I write in the “src” attribute in order to be shown properly? I’ve tried to use the full path (“wp-content/themes/mix/images/rss_a.png”), but that only works when the images are shown on the main page. I guess I could use the entirely full path (“mydomainname/wp-content…”), but I’d prefer a domain-independent method.

Thank you in advance,
Weasel, the Geek