In an academic text, references or other people's ideas should be clearly stated. The references is what distinguish the academic text from other types of texts. By citing other people's work, you will show on what research your work is based and render credibility to your own arguments and ideas. This gives the reader the possibility to value, read more and refer back to the material you use.
It is important to cite the original author to make it clear where that idea came from. To avoid plagiarism, enter a reference as soon as you use other people's research results.
If you want to use a picture, figure or table that is not your own, you need to know that you have permission to do this. Most pictures are protected by copyright laws and cannot be used without consent from the copyright holder. By using for instance the advanced picture search in Google you can filter your search to pictures under different Creative Commons licenses. These are then free to use as specified by the license. For a proper picture reference you must still always specify where you found the picture and the creator.