It is OK to post pictures from other sites but you must say where you got them or it is considered plagiarism. This image has been slightly altered but you haven't made it clear that you didn't take the picture yourself. You can get kicked off this site for plagiarism.
To not plagiarize You can tell where you got the picture directly: http://flashlarevista.com/content/animated-screensavers-download-free.html
or you reference it by using a text link (the code looks like this):
<a href="http://flashlarevista.com/content/animated-screensavers-download-free.html">Free screen savers</a>
so it looks like this in your post:
Free screen savers
or you can make the picture a clickable link (the code looks like this):
<a href="http://flashlarevista.com/content/animated-screensavers-download-free.html"><img src="https://s-media-cache-ak0.pinimg.com/originals/cb/e3/0d/cbe30d88463b714f670437f2d3af82cb.jpg"></img></a>
Which looks like this in your post:
