Only if you do not wish for people to pass your picture off as there own. Online pictures..... will never be printed (most are too small) so they could post your pic on there site and past it off as there own. I dont worry about it for mine. I dont care if some one wants to download one of my pictures, if they past it off as there own, then I guess that means they liked it If there were some that I was planning on selling, maybe then I would put I water mark, but....
This is me, I find most people would not like there pictures to be "stolen" so, well, there you go.
As per above, it depends mostly on how much you want to safe-guard your works. There are of course a variety of ways to protect a digital image from visible water marks to non-visible, amongst other web-development side protections. Generally speaking, all my web-side images get a visible and non-visible watermark. Which is perhaps overkill but it has at the same time saved me a couple of times from image theft.
I'd say at the very least, give it a visual water-mark, your name or something in the top left corner (or anywhere really). Once you've learned how to do it in your respective photo-editing program, it quickly becomes seconds nature. By the way, don't forget to keep a seperate 'clean' copy without the visual water-mark! You may need it!
"plagiarism is the best form of flattery" .... for real, there's not much people cant do in photoshop to get rid of a signature on a photo, espically if its in the corner somewhere... i used to care a lot about people "high-jacking" my stuff ... but now, hell .. if they like it, i might even donate it to them
i've put some photos on a website that people can download high res images for free ... it depends on how passionate i feel about the photograph.. sometimes i like the photo so much that i feel like it needs to be shared, it would be a shame to just have it on my desktop where no one can see it (defeats the purpose of me taking the photo)... just getting my photo out there and my name recognized is sometimes enough for me .. it gives me the ultimate high (ok, ego trip) to know that someone in Europe (whom i never met) has downloaded my photo and used it for their website, desktop pic or a brochure... (only the photos that i uploaded to share) ... how cool is that?
for the people that would steal and photo and claim it as their own .. they know deep down in their heart of hearts who took that photograph .. therefore it would not be self satisfying