You need to make sure you have permission to use any image in your own stories. If you've paid for an image from a stock image library or taken it yourself then you should be OK (as long as any people in your photo are OK about you displaying them in your content).
However, this means that you CANNOT simply Google search for and download an image because you do not own nor have the owner's permission to use it.
Here are some places to find free images:
POWER TIP: If these image libraries give you an option, download images as 640px wide.
Here is an article comparing and testing these sites: Review of 7 free image libraries
Please always check and comply with the usage/permission info included with each photo (eg some may require you providing the photographer a mention). Most of the sites above do not require anything but the Creative Commons search engine images may need something.