There are freely licensed media resources (also public domain) that can be used as sources for file upload at Wikimedia Commons. However never blindly upload everything from these pages. Please always double check if the media is really freely licensed and if it is useful in Wikimedia Commons. Thus it is required to act according to the following points.
Before you upload in the Commons any media from these sources:
Check if it is already available in the Wikimedia Commons - See the search page. Duplicate images are generally detected automatically after upload.
Never forget to include a good description, credit to the author, source and the right license tag (and justify, e.g. with a quote or link, why this license tag is right) - this means you should fill out all the fields in the Upload Wizard form. See project scope.
Please don't data-dump
Not all resources are useful for Wikimedia projects. Examples of non-useful resources include: images without informational value (random party pictures, minor artwork without informational purpose), most content that is biased or flawed. See project scope.
Before you add a source link
Check if the website with the link has a fair amount of free licensed content with a usable license.
Free content search engines and indexes of mainly Creative Commons licensed material. Be aware that unacceptable non-free Creative Commons licenses are usually included in many search engines as well.
Please also take a look at the bad sources that are not acceptable for Wikimedia Commons in order to avoid common mistakes.