As a photographer I deal with this all the time, tracking down those who are linking to my images. Tolouse is right regarding copyright. You have to watch out which websites you upload your images to, as some grab rights from you.
Needless to say copyright won't stop people from copying your image, saving it onto your computer, etc. If it's posted online, even if there are "protections" that prevent right-click saving, as long as it's displayed on a screen people can take it!
And DPI is meaningless for protecting images...if you post an 8mp image at 10dpi thinking it will stop people from producing a quality print think again...anyone can change the DPI at any time. It's best to keep an image small in terms of pixels...1024x768 is the largest image size I would post online, but usually smaller.