This is only a cosmetic bug, but very obvious: when Safari 12.0 in macOS Mojave 10.14 is downloading an image, it can make a real mess of its preview.
This typically affects larger images, which take longer to download. Instead of showing steady progress with increasing image size, when the download is about halfway through, Safari suddenly fills the rest of the image area in with this ‘smear’ in which the last row of the image is repeated identically for every row to the foot.
Once the image has completed downloading, it is shown properly in the browser window.
I had not seen this in earlier versions of Safari, nor in Safari 12 running on Sierra or High Sierra. As it’s only cosmetic and doesn’t reflect the contents of the image as saved, there is no need for any workaround. But it is seriously disfiguring for a while, and I’m sure would have upset John William Waterhouse (who painted the original work mutilated above).