Images on the net are NOT fair game to take for articles you write. Breach of Copyright is a serious business and is illegal.
Most online writers are expected to provide/find images to accompany their articles.
If you are serious about writing it is worth it to learn how to cite images if you are getting them off the net. It is professional practice.
It is fantastic that there are plenty of great images writers and bloggers can use for free. Once you've learned how to search and cite imagery.
Some places for free images are listed in the Self Avenue FAQ.
When you have the image you want to use:
Look for and read the licence/terms and conditions notes.
If the license is 'All Rights Reserved' - leave the image alone.
It is polite to state the photographer/artist name.