What is a legal and allowed method of getting a picture from Internet to be used in the cover of my thesis? I need a picture from the Internet of micro bacterium which I want to use on my Thesis cover. Now just coping the picture and using it will be like stealing someone else's property. Hence I want to know if there is any proper and legal method by which I can get the picture. This is the picture that I need, I wrote them a mail too but no reply yet.
You can ask the author of the image, or the owner of the exclusive licence if that is a different person, for permission to use the image. He may or may not grant it.

If he grants you permission for free then it is courtesy to write an acknowledgement to that effect somewhere in the thesis.

If the image is in the public domain then of course you are free to do with it as you will. (By "public domain", I mean, free of copyright restrictions by the explicit choice of the author or because the image is so old it can no longer be subject to copyright. I do not mean, simply the image was found in public.)

However many institutions have precise binding requirements for theses which would not allow you to place artwork on the cover (even if you already had licence to do so from the author). You should confirm with your institution whether this would be allowed.