Yes. Open an item to edit it and look for the Image section. Select Click to upload a photo to choose a picture from your phone or computer, or paste a link to an image (it should start with https) into the Image URL box under it.
The Add Custom Item form doesn't have an image box, so add the item first and then open it to give it a picture. If you paste a product link under Auto-fill from URL and select Fetch, the product image is pulled in for you.
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