APA 6th: You should always cite images used in your paper according to The Citation Compass. The source should also be included in your reference list.
It's important that you always follow the guidelines given in the reference style you have chosen for your paper.
Colourbox: Image database with illustrations, vectors and videos. Student and employees have 30 downloads each month for academic purposes only. Use your USN username and password to log in and create a personal user account and to accept the terms (see the instructions)
ImageQuest: Contains close to 3 million images. You can use the images in your papers as well as in presentastions in a classroom setting. Click "help" in order to get a guided tour of the database.