if someone posts a picture on a photo sharing site may i use it?

NO, off course you can't, not without permission of the photographer (copyright holder).

Would you expect to "use" a car just because it was parked on a public street or car park?

COPYRIGHT

Copyright applies to all visual arts - drawings, paintings, digital art, photographs, etc. If you did not create the original artwork, you are not the official copyright owner.

According to the copyright laws, the copyright owner has the exclusive right to do and to authorize the following:

1. Reproduce or make copies of the artwork

2. Prepare derivative works based upon the original artwork

3. Distribute the artwork to the public by sale, rental, lease, lending, or transfer ownership

4. Display the artwork publicly

If you are simply copying the artwork for your personal enjoyment and only showing it to family and friends that is ok, BUT if you are reproducing the artwork in numerous quantities, creating derivative works, selling the artwork for money, or letting the public view the artwork (such as posting the artwork on the internet, etc....) that is copyright infringement and the copyright owner has the right to take legal action against you. Check out the copyright office official website for more information! Google search the words "U.S. Copyright Office" to find the website.

Another point to consider is, will your artwork really be a copyright infringement???? It is ok to use other people's ideas/creations for inspiration in order to create your own original artwork. But remember, your artwork can't be a derivative, in other words, will people be able to tell that is "such and such" artwork you copied? Will the artwork be similiar to the original artwork? If so, then it would still be a copyright infringement.

If the artwork is in the "public domain" you can recreate it without penalties. How artwork becomes in the "public domain":

1. The artist forfeited their intellectual property rights and is allowing anyone to use their artwork.

2. Intellectual Property rights have expired. Once the artist dies, tack on an additional 50 years or 70 years (could be less or more years, different countries have different rules, will need to find out what country the artwork was copyrighted in). After 50 years or 70 years have passed, the artwork can be used by anyone. CAUTION: After the artist dies, their estate or heirs might take over property rights which would prevent anyone from using their artwork even though 50 years or 70 years have passed.

fhotoace is wrong about how it Fair Use would apply. This only applied if you are using a copyrighted work for the direct purpose of education. For instance, a teacher could read a passage from a book in a literature class or or could use a photograph to demonstrate a specific technique in a photography class under the fair use doctrine. What you are describing is called a derivative work, and it makes no difference if you are creating it for a school assignment or for sale. Either way it is a copyright violation if you do so without the original creator's permission.

Not without the owner's permission. Just because it's on a photo sharing site does not mean that it's public property. It's still copyrighted by the person who took the photo.

No. You may not. All of Picasa's photos are copywritten to the respectful owner.

You would need to read the terms fo service of the site. Some sites specify that by uploading your pictures to their site, you are agreeing to allow any user of the site to use your pictures.

So, you may *legally* be allowed to use it....

However, that doesn't necessarily mean it's "right" to use it.