1) When you are the user of somebody else's work...
When you use somebody else's works, pay attention to the information like Creative Commons licenses, which tells you how you can use their works without obtaining permission.
2) When you are the creator of your webpage/website...
It's important to communicate with visitors of your webpage/website to let them know how they can use each image/object/file that you made available online. Creative Common licenses are commonly used, and here is the chart.

The original source: Research Outreach Issue 105, pp. 6-9.