Just look in the “images” files for that logo. Replace it with one of your own, but name it the same thing as the original footer logo. You can even use the original file as the basis for creating the new one in Photoshop or a similar program.
Note that the logo can go in any of three places, depending on which color variation of the theme you are using…in the root “images” folder for the template, or in the color subfolders. In a case like this, I simply put the modified image in all three places so it will appear no matter which color variation I choose. However in my case I don’t want to vary the logo’s color. If I wanted it to be different for each color variations, I would create variants and put them in the appropriate places.
Hope this helps.