The linked video didn’t show the part where he got the text for the dialogue speaker, but wherever that happens, just past an image along as another input (probably from a var stored on npc) and set the image in the widget.
To implement this, you need to create a variable of the type Texture that corresponds to the speaker’s name and dialogue. Then, pass the speaker’s icon to the appropriate function (e.g AddDialogue).
On the widget side, you can retrieve the texture and use the function to assign it to the Image component you have already added to your widget. You can use SetBrushFromTexture