This is a minefield and there are many here who know more about it than me! My advice, if you want to use clipart is to look at a site like Clipartopolis which has free downloads.
For any site which offers clipart or fonts, you must read their terms and conditions very carefully.
Please make sure you visit a reputable website and never buy Licensed designs like Disney or other well known characters.
As spendlove mentioned it is imperative that you read the conditions of use on any website. Often if you want to "change" the picture by digitizing it, you must pay extra. Also, some sites will have you pay more if you plan to sell the designs.
Some sites offer digital stamps for card or scrapbook makers, like www.bugaboostamps.com. But you have to read the terms and conditions (or terms of use) as some do not allow the artwork to be digitised.
Most embroidery websites state where they got their artwork. If you like the style of a design have a look at the site the artwork was obtained from.
Use your own photos (and drawings)