If a dead creature which does not have flowing blood falls into some water, it does not make the water impure. The scholars say this includes a gecko and anything smaller than it.
The gecko came to the fire in which Sayyiduna Ibrahim had been cast and blew upon it, trying to increase its ferocity. Obviously this had no effect on the fire but due to its bad intention it is recommended to kill geckos.
The frog on the other hand filled its mouth with water and put it on the fire, trying to extinguish it. Obviously this had no effect on the fire but due to its good intention and its respect for Ibrahim it is impermissible to kill frogs. Continue reading Insights into Purification: The Gecko and the Frog