Just because you have found an image freely available on the internet, does not mean it is legal for you to use it. Generally, under copyright law, unless you are the rights holder, you must have permission to use any material you find.
However, it is recognised that there are times when it is necessary for people to be able to use material without having to seek permission from the rights holder every time. Under copyright law, there are a series of what are called 'copyright exceptions' - basically, a list of particular kinds of use that are legal without needing to seek permission. One of these is referred to as the 'examination exception' - there are others, for example, news reporting, use in research, making copies for private study etc. For more information about copyright, please see our Student Guide to Copyright.
The examination exception means that you are allowed to use any material in a piece of assessed work without needing to get permission to use it from the rights holder. 'Assessed work' means any piece of work that will be assessed, either formally or informally, so an assignment, a presentation, an exam, a dissertation. It is subject to something called 'fair dealing', which means you must use the material in a fair manner, i.e. not use any more than is necessary, not use it in a way that may impact on commercial sales, not use it in a demeaning or derogatory manner or in a way that may reflect badly on the creator.
One of the most important elements of 'fair dealing' is that you must acknowledge the rights holder, unless this is impossible to do so, like in an exam, for instance. If your work is visual, audio or film, you must include a separate written sheet listing the sources of material you have used. This requirement to acknowledge the rights holder means you must not only reference your source, but know who the material belongs to in the first place. This can be difficult with images found online, as it is very easy for them to be uploaded by anyone without any detail about who the original creator is.
Because it is an exception specifically for examination, it will not apply if your work is subsequently used in another way. If your work is intended to go on wider display, i.e. beyond yourself and the assessor(s), the exception no longer applies and you must ensure you have permission for the material used or use pre-cleared material. So if your assessed work includes a public performance or if your poster or graphic work will be included in an End of Year Show, you cannot rely on this exception and must ensure you have permission for any material you have used or you have taken it from a legal source, such as one of our subscription databases or material licensed via Creative Commons or public domain.
If the assessed work includes the creation of a web page, this cannot be made available to anyone but the assessor. If it is made available on the internet, all material used must be pre-cleared or used with permission.