OBS has some limitations when doing what you want as it is primarily intended as a broadcasting tool. Many video editors will allow you more flexibility...
You mean, record a *video* that has a static image and a soundtrack?
If that's what you want to end up with, then I agree with John. If you're not broadcasting live, and you don't need tight A/V sync, then OBS is probably not the right tool.
John has two suggestions for a video editor, and here's a third one that *I* like:
Shotcut is a free, open source, cross-platform video editor for Windows, Mac and Linux
shotcut.org
Unlike some other popular ones, it's free and full-featured forever (open source, like OBS is), and does a LOT! But I still think it's easy enough to get started with and grow into.
Record your audio by itself, probably with Audacity (also free and open source):
Audacity is the world's most popular audio editing and recording app. Edit, mix, and enhance your audio tracks with the power of Audacity. Download now!
www.audacityteam.org
Then import both that and your picture(s) into your favorite video editor, build the video in there, and export. OBS is not involved in any of that.