The Icon Library contains a repository of all the icons used in PPO (including action icons such as Add Issue and Edit Task, and other icons such as My Time Entries, Dependencies, Data Fields and so on), as well as PPO-friendly icons that can be used on the Life Cycle.
For full details on how to create or customise the Life Cycle, see the following group of knowledge base articles.
Below is a zipped folder with some icons that you can use. Another option is follow this link to download other icons.