Add a low-resolution (10x10) image that would be stretched over the area where the actual image will load. Make sure the 10x10 thumbnails are only generated once and stored on the disk for fast access later on.
Care should be taken to make the thumbnail image match the position and cropping of the final image.
There's an abandoned diff for this task that could be an inspiration: https://hub.sealcode.org/D1206