Copyright for Students
All items uploaded onto the internet are automatically covered by the Copyright Act (1994) unless a different license, usually a Creative Commons License (allowing you to reuse the item) is shown on the site.
Fair Use (or "Fair Dealing")
It is a bit confusing how the Copyright Act applies to students at school (link to info sheet), but it seems* safe to assume that you can use any items for your school assignments as long as
- you include a reference to any items, acknowledging the owner
- there is only one copy which is only viewed within your school - by yourself, peers and your teachers.
* You need to make your own decision on this. Please check with your teachers to find out how they want you to deal with copyrighted material. Also check the actual task instructions - you should find clear instructions there.
Fair use of copyrighted material allows for a single copy of the material as long as the following is acceptable (taken from Otago Uni website):
- the purpose of the copying (e.g. for some commercial purpose would be less fair than for private study)
- the nature of the work (e.g. whether it required a high degree of skill to produce it)
- whether the work could have been obtained within a reasonable time frame at a standard price
- whether the copying would affect the commercial value of the original work
- the amount copied in relation to the whole
What if there is more than one copy?
If there is any chance that there will be multiple copies of your digital outcome (e.g. if it is uploaded to a public accessible website like Youtube or a shared Google Drive) then the Copyright Act takes effect.
This basically means that if an item is copyrighted then you cannot use it or alter it without the permission of the copyright owner. For full details see these pages: Student Guide to Copyright on TKI and Student Guide to Using Other People's Stuff
Remember that there are other considerations to make as well as copyright (see below).
Best Practice - Creative Commons
Always search for items that have a Creative Commons license. A license which allows you to reuse the items, sometimes with certain restrictions. It is always best to acknowledge the source of the item even when the license doesn't require it. To find ot more about Creative Commons click this button
Summary
Using copyrighted images for school assignments is only if the assignment stays between you and your teachers, otherwise you need to get permission. Always acknowledge the owner.
Next: Creative Commons
Other Considerations
Apart from copyright, and the related topic of "fair use", you also need to consider things like other people's rights (e.g. their right to privacy) and not offending others. Also you need to know how to reference any items used in your school assignments.

Privacy
Respecting other rights to privacy is covered by the Privacy Act. One important part of the act is that you cannot share information about or images of others without their permission.
Image source (License: CC0)
pixabay.com/photo-414227, 5Dec17

Not Offending Others
Moral considerations are very important. The content of your digital outcome must not offend, upset or harm others in any way. This not only applies to the images used, but also the language and information used.
Image source (License: Public Domain)
commons.wikimedia.org/wiki/File:Damon_shocked.jpg, 5Dec17

Correct Referencing
It is good practise to include the source of any item you use from the internet. There are a few accepted ways to do this. Always do it in such a way that it does not effect the design of your digital outcome.
Image source (License:
CC BY Attribution 2.0.)
flic.kr/p/e2QMS5, 5Dec17