Open the wish, click the upload tile in its image gallery, and pick your photo. You can attach up to 5 images per wish.
Steps
Open the wishlist and select the wish, or open the new-wish form.
In the image section, click the upload tile.
Choose an image file, or drag one onto the tile.
Save your changes.
Attaching a photo vs. starting a wish from one
These are two different things. Attaching a photo adds an image to a wish and changes nothing else. The name, price, and link stay exactly as you set them.
Drop a photo into the Add Wish bar instead and Wishpicks opens a new-wish form with that image attached and suggests a wish name, plus an optional description, for you to edit. It never touches the price or buy link. See "How do I add a wish from a photo?"
Good to know
Once you're at 5 images the upload tile disappears. Delete one to make room for another.
Allowed file types are JPEG, PNG, GIF, WebP, AVIF, SVG, and BMP, and each file has to be 5 MB or smaller. Anything bigger is turned away with a "too large" message.
The first image is the main one. Drag images to reorder them, and whichever sits first becomes the wish's primary picture. You can delete any image on its own.
Most links bring a photo with them anyway. When you add a wish from a product link, Wishpicks usually pulls the product image in for you; see "How do I add a product from another website?" Uploading your own is useful when a link didn't bring one, or you want a clearer shot.
Editing a wish on a shared wishlist needs Contributor access or higher; see "What is the difference between viewing and editing access?" in the Shared Wishlists collection.