Changing start button

Hello, I'm trying out Zorin OS for the first time. I noticed I can change the Start menu button and thought I would play with that along with other customization options. When I right click it and go to the Preferences menu, I click Install and try to select a graphic to use in place, but the graphics I have on the machine are not visible as selectable options (.png, .gif, .jpg). I assume it's looking for a different file type, but I'm not sure what that is and I have had trouble finding documentation on it (though I could just be looking in the wrong location).

Thanks for any help you can provide.

Thanks for the video. If I have a particular graphic I am aiming to use for the start button, how do I select it? I'm clicking the Install button next to the left of the dropdown you start clicking around 0:51. Is it looking for a different file type, like a .tiff?

Thanks for the info. So if I have an icon set to install for the start button, do I need to create a folder for them in the Buttons directory? Nautilus doesn't let me create a new folder there, so I assume I need to have root privileges. Can I work through Nautilus with root privileges or do I need to do it through the terminal?

I know this isn't essential, I'm just enjoying playing with the customization options and this one still gives me difficulties. Thanks again for the help.