First and foremost, thank you for the reply. It's very much appreciated.
Just for clarification, I am using vBulletin Cloud.
The Pelican logo is part of my header site logo. It is all one, continuous image. The 2 logos are not separate files. I joined these together in Adobe Illustrator and saved them as one file. When I first added this logo file to the header, it wouldn't display properly. So I went in to the Edit Site section on vBulletin cloud and there is a STYLE tab. I clicked on that and changed the max-with and max-height of the site logo CSS to 100% so that the single image would display across the entire section where the site logo goes in the header. See image.
However, when viewing from mobile devices, this image doesn't respond as I'd like, so I think I need to utilize a different image or set of images altogether to replace the existing site logo. I imagine I would need to use some kind of CSS to keep the current site logo hidden on mobile, and then use media queries to assign a different site logo for mobile. My skillset just isn't there to tackle this. I'd like to have the 2 images stack one on top of the other, as such.