When re-using someone else's work, providing credit to the original creator is good professional practice. The aim is to provide sufficient information to find the original and to make it clear under what terms you are using the work. The TASL method of attribution is a common approach:

Author

Picture of the letter A

"Letter A" by Kapungo is licensed under CC BY-NC 2.0

Source (link to the original file)

Picture of the letter S

"Letter S" by Double--M is licensed under CC BY 2.0

Licence (link to the licence deed)

Picture of the letter L

"Letter L" by emmacraig1 is licensed under CC BY 2.0

For another good example of how to attribute work from others that you want to include in your own work see this from Saylor Academy.

For guidance about licensing other people's work where you have made modifications (adaptations or derivatives) or created a collection of work see adaptations and collections.