When attributing a source there are three main elements to include: the title of the work, the creator’s name and a link to the original website where the work can be found. You may also, for an image for example, include the licence under which the image is shared.
To remember full details of what to include when attributing, you can use the useful acronym TASL:
T – Title of the image/material included
A – Author(s) or creator(s) of the image/material
S – Source of the image/material (where it is from, including a link to the website if possible)
L – Licences (show which of the Creative Commons licences applies to the work)
All Creative Commons licences require attribution. For more guidance on how to attribute material correctly visit https://wiki.creativecommons.org/Best_practices_for_attribution
Take a look at how the image below is attributed to get an idea of how to include all of the necessary information:

Strawberry pancakes by Flora M, licenced under CC-by-2.0
https://www.flickr.com/photos/101616559@N07/43318832054/in/album-72157689514709075/
If you’re feeling creative and are using your own images or drawings, you can also attribute yourself! A simple "Images created by [yourTESshopname / yourfullname]” will suffice. If you’d like to learn more about licencing, you can browse through the creative commons network https://creativecommons.org/.