I released the following imagines under a Creative Commons Attribution-ShareAlike 4.0 International Licence.
What does this means?
It means that you can freely copy, share, adapt the image for any porpoise as long as you give appropriate credit to the original author. All the info about the licence are provided at this link
The imagines can be exported as png files from the following list, the vectorial format files are also available (.svg) by clicking on the images
For more free images you can check my list on wikimedia commons.