Plagiarism is wrong and if you dont know how to cite something you got text from, then just ask someone or look on the Internet. You could be arrested, or in school, fail.
The way to tell where they got a picture from (yes I did use this to cheat the picture game) is to right-click it and press properties. It'll show you the coding for the picture and most of the link it was derived from. You can also press copy image location and it will show you where it is when you paste it.
Problem - some people are too lazy to click links.
Solution - Copy and paste only the RELEVANT information, put it in QUOTES to show you are quoting someone else, and post the link to the main article.