The images are great for using in PowerPoint presentations to give it a professional look and feel. Using a high quality collection like ScienceImage to source images from will ensure you don't waste time trawling through clipart or armature images from Google. All you need to do is ensure you attribute the image correctly by adding a caption under the image and if required by your lecturer adding it to your referencing list.
Here is an example of attributing the CSIRO in the image caption:
|A beach in the Northwest Shelf - Image from CSIRO ScienceImage licensed under CC BY 3.0|