Unfortunately, as of January 2025, there no native solution to set an image as a complication. To use custom images, you will need a complication source to supply your desired image. I recommend Photo Complication for Wear OS from amoledwatchfaces as you can select multiple images to be shown in a slideshow.
Once you have Photo Complication for Wear OS (or a similar app that adds photo complications), you can perform the following steps to set your new background.
Hold down on the watch face
After holding down on the watch face, you should click the edit/customize button.
Alternatively, in the Samsung ‘Wearable’ app, it looks something like this if you have Photo Complication for Wear OS installed.
🎉 Yay! You should now have your custom background!
You might notice that your images don’t look quite right when you port them over to the photo complication app. To change this, try cropping your image to a square aspect ratio manually before sending it to your watch.