VTOY_DEFAULT_IMAGE not working?

[longpanda]

OK, thanks - that is not mentioned on https://www.ventoy.net/en/plugin_control.html

Also another issue...
If I select (for instance) the fourth folder in a menu, then the fourth ISO menu entry in that folder is automatically selected and not the first menu entry.
If I select the 7th folder, then the next menu has the 7th menu entry selected, etc.
Can each new menu start at the first menu entry?

Yes.
By default, when you enter a sub-directory, the 1st menu entry will be selected. 
I modify it for a better experience when press ESC to return the parent directory.
For example, when you return back the 7th folder will still be selected (by grub2 default, 1st will be selected).

I will try to fix it. If can not be fixed, I will provide an option in control plugin.