milk already gave you the solution.
You need to setup an ImagePoint on the running sprite where you want the splash to be positioned. Also remember to check the origin point of the slashing sprite, these points will overlap when you create the sprite.
Post a capx if you're still having trouble
That's so weird, I must be doing something wrong.. I hope no one minds walking me by my hand on this...
1. I make an image point, called Sword, and then place it on the belt buckle of all the "running" sprite frames
2. I set the "Slashing" sprite's origin on the belt buckle as well. Then I give the sprite a "Pin" behaviour.
3. I set that the "Slashing" sprite would spawn at the image point "Sword" whenever the player is running and attacking at the same time. (I also make the 'slashing" sprite pin to the "running" sprite.
4. I play test it. In theory it should work, but nope, the "slashing" sprite spawns on the "running" sprite legs.