Vector art is very different from bitmap art which is a photo or an image that can't be resized. Vector by contrast is made up of lines and points, so the program you use to edit it can change its size without getting jagged edges or making the artwork fuzzy. This is especially important if you're creating flyers and posters and want different sizes.
The type of clipart that I use is important for all of the different effects that I'm creating. It needs to be a vector drawing for sure, as non-vector drawings are bitmaps that don't resize nicely... they'll just get all bumpy and not look good. Also, I'm drawn to simply line art because if it's too complex sometimes it doesn't look as good in the different sizes. It's also good if it's a single color, usually black, because then I can change it to whatever I want quite easily just using simple graphics software. Just remember, simple is better, is cleaner, and usually creates a more sophisicated look.
Another interesting place to look at for your image needs is Microsoft Office ClipArt in the Microsoft Office Program. As per the EULA of the Office Program, you can use these images in any document or for any work as long as you are not distributing only the image. Microsoft also allows access to its huge database of quality clipart and graphics at office.com. These images are of high resolution and some are even in vector format. You can download images and use them in your website or blog without a hassle. The Office.com Terms also mention that you can use these images in any document, even if it is for commercial purposes.