How to copyright an image which will be posted on a tumblr blog

Unless you live in one of the handful of countries that are not signatories to the Berne Convention, I can reliably state that you automatically get copyright protection on your works when you create them.

Enforcing that right can be a lot trickier and some countries give you extra privileges if you explicitly register your copyright with the government, but having your works "copyrighted" is automatic.

If you mean something else by "want my photos to be copyrighted", you'll have to elaborate. (Displaying copyright notices? Licensed under terms other than "All rights reserved"? Something else?)

UPDATE:

If you took a photo, you can do whatever you want. Every way is "legal". Copyright is about letting you control what other people can do with your creations.

Watermarks are one way to ensure attribution for your work, but if you actually want people to reblog them, stick to something small and in a corner or along the bottom. Big watermarks are annoying and, with so many other people using Tumblr, if you're annoying, people will just look elsewhere for their pics.

Another method that you can use in addition or instead, if your photo software supports it, is to embed XMP metadata into the photo, which will let other programs automatically find and use the authorship information. (Think of it as similar to the ID3 title/artist/album information in MP3s, but for pictures instead. You've probably run into at least one MP3 which showed up as "Untitled" or "Artist - Track 6" in your playlist because it had no ID3 metadata.)

As for Creative Commons licenses, they're a way to pre-emptively grant permissions that you're comfortable with, so people don't have to ask you for every little thing. (This makes people more comfortable in using and sharing your photos, limits the harm if people ever find themselves unable to contact you, and may make some people more willing to stay within the rules you set by showing them that you're a reasonable person who deserves some respect.)

If you follow the instructions in the Creative Commons licensing tool properly, it'll also make sure that your photos show up when people search Google for only images with creative commons licenses.

Tumblr doesn't appear to have any special functionality for automatic watermarking or Creative Commons license marking the way sites like deviantART or Flickr do, so all I can suggest that's actually on-topic for webapps.stackexchange.com is that, if you want to put your entire Tumblr blog under the same license, you can follow the instructions Creative Commons offers at http://wiki.creativecommons.org/Blog/Publish#Tumblr