Add clipart images to your photos with the clipart tool. To find this tool, click the Clipart icon inside of the More Effects dropdown in the Effects tab of the toolbar.

Use the panel on the left to select a clipart category. When a category is selected, a list of clipart images will be displayed in the center of the dialog. Click on any image in the list to see a preview on the right side of the dialog.
When you find a clipart image you like, click the Download button. Once the image has finished loading, the clipart image will be added to the project as an Overlay Image layer.