Copyrighted material may not be used in master's theses without permission. Such material are for example different kinds of pictures, drawings, maps and photographs (Copyright Act, Section 1)
However, works of art made public and permanently placed at a public place may be reproduced in pictorial form with a text in a critical or scientific presentation (Section 25), when the picture has a material connection with the text and it is used to illustrate or clarify the text.
Begin the acquisition of permissions in good time, preferably immediately when you find a picture that you want to use in your thesis. If you get no reply to your permission request or if the reply is negative, do not include the picture in your thesis.
You can usually begin requesting a permission by contacting the publisher.
Most publishers offer a service with which you can request a permission and it is granted immediately, for example RightsLink. If such a service is available, you are advised to use it. For online images with no copyright information available contact the webmaster in question.
You can use the attached template for requesting permission.