Menu in mobile view

Hi, yes, we can do that with the following CSS.

.site-navigation h1.menu-toggle::before {
    content: url("URL_OF_IMAGE");
}

Replace URL_OF_IMAGE with the image you wish to use. I would suggest uploading it to your media library. For reference, the existing hamburger icon is about 20px square.

If you have not yet done so, I would suggest creating a child theme so that any customizations you make will not be overwritten by a future theme update. Alternately you can use the Edit CSS function in the Jetpack plugin, or use one of the other Custom CSS plugins that will keep CSS changes from being overwritten.

Child Themes
Child Theme creation plugins
Jetpack plugin
Custom CSS plugins