As there exists a "screenshot" variable in a theme's .info file, there should also exist a "logo" setting so theme builders can include a default logo other than "logo.png" (I typically use a jpg for photographic logos)
e.g. .info file
name = MyTheme
description = My custom theme
screenshot = differentscreenshot.png
core = 8.x
logo = logo.jpg
settings[toggle_logo] = 1