How to get caption to Javascript Add-in in Web- and Windows-Client

Hi,

I'm "playing" around with JavaScript Add-ins in NAV2016 and created a simple add-in that Displays an image on the Customer Card.

Now I'm facing two problems:
1. Allthough I've set up a caption in all languages it doesn't show up in the Card, not in Windows Client and also not in Web Client
Do I have to tell the add-in that it should allow captions on the page or do I have to do it manually via html-code an JavaScript?
I didn't find any add-in or blog with captions today.

2. My Image is correctly shown in the webclient (ok without caption) but not in Windows Client
There I only get a black cross/placeholder.
In my JavaScript file I only did this:

function ShowBar(id) {
    $("#controlAddIn").append("<img src=" + Microsoft.Dynamics.NAV.GetImageResource(id) + ">");
}

How to get the Image shown in the Windows Client?

Thanks in advance for pointing me into the right direction.