Sprite Sheets

So I'm putting sprite sheets into practice now since we're making/experimenting with animating in unity 2D! I've learnt a good bit about it already, after doing my initial one I found out I need to use a bigger sprite sheet in Photoshop to make more room for the animation and I also preferably want more than 10 frames.

That's the 10 frame one ^^ 

Now I'm working one one with 15 frames and twice the height and a quarter the length... problem being now I cannot save them as a PNG! 

So I'll need to talk through my options with team mates on Thursday!