To get a different logo image for the Fixed Navigation header (when you scroll the page) simply add the following code to the Custom CSS field in WP Dashboard > Divi Theme Options > General Settings > Custom CSS (at the bottom)

.et-fixed-header img#logo {
    content: url('http://icons.iconarchive.com/icons/google/noto-emoji-animals-nature/1024/22221-cat-icon.png');
}

Don't forget to define your own logo image inside the single quotes. So, you'll need to replace this image URL:

http://icons.iconarchive.com/icons/google/noto-emoji-animals-nature/1024/22221-cat-icon.png

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