The Horizontal Box allows “many children”. For example, it can contain a button that the player can press to select the item and an image that would lay out horizontally after the button to graphically represent the item. In the Hierarchy pane of your UMG, it would show like:
Horizontal Box
Button0
Image0
To put your image into the Image Widget, use Make Brush From Texture and Set Brush nodes. The Target will be your Image Widget and the Texture will be your graphic. Be sure “Is Variable” is checked so you can programmatically access the respective widgets.