Where can I find free to use pictures of an explosion sequence? (similar to those used in the GD examples)? I usually use Pixabay for images but can’t find any.
http://opengameart.org is a good place, though there should be more good sites out there. Read carefully, generally you can use the images for any purpose (e.g. commercial), and sometimes you can’t, and almost always you have to give credits to the autor in specific ways that this autor indicates (his/her name, a website, etc.) 
Thanks