There are at least three good reasons to cite sources in our digital age. Here they are listed in increasing order of importance.
And I believe the least important. Now, that is not to say unimportant. We need to cite sources to obey copyright law. Most trouble with copyright can be avoided by appropriately citing sources. Teachers often point to Fair Use Copyright doctrine or the Teach Act 2002 as valid reasons for using information, but this does not excuse proper citing of sources.
I think the second most important reason for citing sources is: Modeling good behavior for students. In our modern day with plagiarism and copyright violation so easy, it is easy to take the approach as a teacher of just using what is necessary to get the job done. After all, it's all out and available for the public anyway right? Well...sort of? Information that is found in 3 independent sources can be considered general knowledge and is exempt from the necessity of citation. The challenge with this is that items of a multimedia nature are not always public domain, even on government or other websites since the work may have been commissioned and the images or video may be copyrighted materials, subject to licensing and royalty agreements. We should cite sources to provide a good example for students of giving proper credit and not presenting others work as our own.
And I really think this is the most important reason! We should cite sources properly so we can find the source again should we need to. Ever accidentally deleted a photo out of PowerPoint? Have trouble finding it online again? If we cite our sources well we can find these resources again. Some may not be essential, if all you need is any picture of a giraffe it is fine, but if what you need is a picture of a giraffe and you lose it, having citations can save the day.
So, how do you navigate this jungle in a digital age?
A couple of suggestions
Understand the difference between formal and informal citations. Formal are the tried and true citation formats for MLA, APA or other defined styles. Informal are just that, informal methods that accurately reflect where information was found but don't follow a formal style sheet. A tendency on the web for citing sources such as flickr, youtube and picasa is to cite the user: i.e. photos from flickr user: covili
For more formal citation help try: Citation Machine