Hmm I had the same problem. I created 11 images (0-9 and off) and when I tried to load them into the B2S Designer it would only ask for 10.
I'm not sure if this was on intention or bug.
The message clearly says 0-9 and off, but the code has a commented/removed message about off, and does not handle it at all.
Either it was delayed for later (and forgotten), or removed on purpose (maybe because of bugs in the Server itself).
I suggest, before we change the designer code, maybe you want to test if it will work with the B2SServer without side effects.
Create your backglass with your own LEDs, 0-9.
Open the .direct2BS file with a text editor.
Scroll till you see:
<Reels>
<Images>
<Image Name="ImportedLED_T1_0" CountOfIntermediates="0" Image="iVBORw0K....YII=" />
The 'funny text' beside Image= is the base64 encoded image.
The line is repeated 10 times, from _0 to _9
Just for testing, add as 11th line this one:
<Image Name="ImportedLED_T1_Empty" CountOfIntermediates="0" Image="iVBORw0KGgoAAAANSUhEUgAAABQAAAAeCAIAAACjcKk8AAAABGdBTUEAALGPC/xhBQAAAAlwSFlz
AAAOwwAADsMBx2+oZAAAABh0RVh0U29mdHdhcmUAcGFpbnQubmV0IDQuMC45bDN+TgAAABhJREFU
SEtjGAWjYBSMglEwCkYBVsDAAAAHJgABxtzEJgAAAABJRU5ErkJggg==" />
The image is a black rectangle, just for testing. of course you might want to use your own image (base64 encoded)
The trick is, as you can see, to add as 11th image one with the name _Empty.
In a quick test it seems to work, but more testing would be required before changing the designer (enhance the loop from 10 to 11, save the 11th as _Empty).