There are several ways to put a new element on a label. Whichever you use, the new element is selected straight away, and you can undo it.

Ways to add an element
From the element library	Click Add element in the toolbar, or right-click a label and choose Add element. Then double-click an element in the element library, or drag it onto a label. A double-click puts it in the middle of the label; a drag puts it where you let go.
Drop a picture file	Drag a picture from a folder onto a label: it becomes a new element with that picture, as big as the picture and no bigger than the label. Dropped on the page, away from the labels, it becomes a page element.
Drop a field	With data connected, drag a field from the Data source panel onto a label: it can become a new text, barcode or picture element showing that field. The field has to go onto the label set its data belongs to.
Paste	Copy an element and paste it: see Copying, pasting and deleting.
Ask Joy	Tell Joy, the AI assistant, what you want - "add a barcode with the order number" - and Joy adds it.

With more than one label set, double-clicking in the element library doesn't know which label set you mean: drag the element onto the label you want instead.

Picture files can be JPEG, PNG, WEBP, GIF, TIFF or BMP. For a picture on an element you've already got, use its Image section.

What a new element looks like. The factory default document, and a new label set made with Default set, start with three: a text, a picture and a barcode. An element from the library starts as the library has it, and you change it in the Label element panel.