Hrm. I'm very cautious when looking at "free download" sites like that. They typically just grab content from wherever they can and use it to generate site traffic. Use it if you will, but there's really no clear indication of who the creator is or how they're granting license to use the image.
OK I'm going to use it. I don't my peers will think I should be disqualified for that particular gif. If anyone would DQ me for using this, please post here and I will do something about it.
At least you know who made them and how they are licensed. Oh well.. I won't DQ you for one gif but you may have a look at OpenGameArt if you need other graphics.. ;)
http://www.positech.co.uk/content/explosion/explosiongenerator.html
If I remember right, the only requirement for using it is acknowledging him in the your game's readme.
http://www.ecybers.com/webdesign/images.htm
"Note: All images were either created by me or downloaded from the web sites that claimed they were free to use."
This wikia page claims that the gif came from the above web site.
Am I clear?
http://opengameart.org/content/explosion
http://opengameart.org/content/asteroid-explosions-rocket-mine-and-laser
http://opengameart.org/content/explosion-with-alpha
At least you know who made them and how they are licensed. Oh well.. I won't DQ you for one gif but you may have a look at OpenGameArt if you need other graphics.. ;)