I am not a lawyer. This is only an 300-word overview of issues to consider. The article on copyright at Wikipedia has more guidance and references.
When using text, photos, artwork, or music by others, one must consider copyright.
Copyright is granted automatically on creation of the work. There is no need to specify a date or copyright holder. By default, all rights are reserved and are exclusive to the creator of the work.
To use the work needs permission from the copyright holder, not necessarily the original producer or creator. A licence may limit the use of the work.
When there are allowed, or defensible, uses of copyrighted materials without permission, these are often not well-defined.
Is copyright clearance needed worldwide or only where you intend to publish?
“Do I have permission to use it for this?”
You read the license terms carefully, and your usage is allowed, but are you sure the licensor has the right to grant that license? Most licensors disclaim responsibility for this, even if you pay for a licence, and oblige you to indemnify them if problems arise.
Trademarks, moral rights and international treaties multiply the uncertainties.
Images that include identifiable persons or private property have additional constraints, perhaps needing model and property releases.
“Is it still in copyright?”
Copyright lasts for many years, often more than a century. Most copyright terms are not predetermined but expire some years after an event such as the death of the creator.
Jurisdictions extend copyright terms retrospectively from time to time, bringing out-of-copyright items back into copyright.
A third party may control the copyright.
A work is orphaned if it is still within copyright but the copyright holder is unknown or not contactable. There have been proposals in some jurisdictions that third parties may be allowed to charge for licences for orphaned works.
Licences can change over time. Copyright owners may release a work under more than one licence or revoke one.
Attributing copyrighted material
Others’ material included in websites should have its source and licence specified in metadata and, where possible, in a caption or citation. Links should be to the origin not just to a search engine or aggregator.
For information about a licensing scheme designed to encourage creativity, visit the Creative Commons website.
As mentioned above, start with the article on copyright at Wikipedia.