There are tools available on the internet which enable you to embed an
invisible watermark in your image. Won't stop someone nicking it, but at least you can prove your ownership if someone
does nick it. It is virtually impossible for anyone to tamper with the image in such a way as to remove the watermark, short of destroying the image entirely.
One such service is available as a plug-in to Photoshop, Paint Shop Pro, etc. It used to be a free service but now, it seems, you have to pay a subscription. See
here. Might be worth it if your copyright is of value to you.
It is supplied as a demo with Paint Shop Pro. The demo version cannot be personalised, it merely embeds the words "Jasc Watermark Demo", so it's no use for protecting my copyright! But it illustrates the principle. Since my
metier is astronomy stuff, here are two almost identical images of the same star cluster, one with watermark, one without. See if you can tell any difference.