Wikipedia:Upload/Some website

You should look at the terms of use or license statement on the website you found the image. Unless they explicitly state that images are under a free license, we can not accept the image. You may be able to use the image under fair use if it applies under our criteria. You should likely seek help before trying to upload the image at Media copyright questions.

Why can't you just take any image?
Wikipedia is founded on the 5 pillars, which the third pillar is "Wikipedia is free content." We do not charge for access, and in order to encourage free information, we want free information. Legally we probably could take just about any image and use it here on Wikipedia, but then re-users of Wikipedia's content wouldn't be able to use most images. If it's important enough to get an image onto the article, you should want all uses of the information to contain that image.