It is best not to use copyrighted images with watermarks.  

For more details, please see a great related article

Free image resources

Never need to use a watermarked image again.

The following websites all offer images that are free for you to use. However, as websites change frequently, I would suggest that you take a moment to check their image usage guidelines. Even though images are free to use, certain sites require you to leave an accreditation detailing the source of the image, so please check and keep to the rules.