GoMultiTextNode: Transparent Icons

how do I show an icon on a GoMultiTextNode with transparent background?
Further, is it possible to hide the horizontal line between to lines of text in a GoMultiTextNode?

That depends on whether the image you specify supports transparent backgrounds. GoDiagram just draws images using .NET’s method(s).
By the way, in case you haven’t seen this already, the RecordItem class in RecordNode.cs in the Demo1 sample supports having a image along with a text as each item in a RecordNode.
GoMultiTextNode uses a GoListGroup to hold all the items and to draw all the decorations, including the separator lines. GoListGroup doesn’t have any support for selective omission of lines. But you might be able to fake it, if you have a solid background color, by drawing a line with that same color in exactly the same place as the line you don’t want drawn.
It might be easier to implement this by using a single list item that holds the two objects you want. Perhaps you could implement this by using a nested GoListGroup that doesn’t have any BorderPen or LinePen and that just holds those two items.