You can choose an image from DesignBold's stock library of +30mil photos. You will find them by clicking "Search" at the top of toolbar >> Free photos if you just want free.
You can select images by topic by typing the topic name on the searching bar.
Choose the photo you like >> Drag it to the right position.

You may want to use your own image to make your shop icon more appealing to shoppers and relevant to real products. To do so, click on "Upload" button on the toolbar.
>> Click "Upload your images"