In the old QuickView, you could save a chart as an image. This works exactly the same in the ReportBuilder.
From inside the Viewer, when you move your mouse over an element, the menu becomes visible. Click it and select Save as image.
On mobile devices, click the element to display the menu. Click it and select Save as image.
If you’re inside the Builder, first click an element to select it. On the right side of the properties pane, open the menu and click Save as image.