Thanks for writing in. I understand you need to replace the video with a still image without affecting the audio. Am I right? Rush does not have this feature now. If you submit it as a feature request here, our product team will be able to review it in more detail: https://adobe-video.uservoice.com/forums/916423-premiere-rush
You can place the still image on the track above the video track. This way it hides the video on the background.