first off, there are some services and applications around that allow people to digitally watermark their work... this embeds information about the producer of an image into the image in such a way that it can later be pulled out. this is true even if the image is resized, etc. also, there are companies (like digimarc) that allow their customers to send out bots (like what google uses) to search for their images being used in untoward ways.
this technology is just gaining steam, though, and there are many competing companies, so i wouldn't be too worried about it...
BUT...
any images - photographs, illustrations, etc. - are the property of the person who took them. a great many people choose to allow people to use their work without permission, however...
if you have found some photos, drawings, etc. that you would like to use in a professional piece of work (website, brochure, business card, poster), it would behoove you to contact the source of that material and just ask for permission.
the only ways you can get source material legally without contacting the originator of the work are:
a) make it yourself
b) go through a service (corbis, getty images, etc.) where you pay for the rights to use an image
c) use the images of someone who explicitly is willing to share their images (through licensing like that of http://creativecommons.org/ )...
d) anyone out there got other legal ways to use other people's work?
the bottom line is that, if someone doesn't give you permission - either through an exchange, an explicit message or personal contact - you can assume that using their images would be a violation of their property rights... and, though they might not go after you, you'll still be stealing from them.