Change default screenshot format for iPhone 6 Plus running iOS 8? I would like to change the format of a screenshot from .png to .tiff. Is this possible? Moreover, is it possible to write a script or some equivalent that can automate screenshots on any apple mobile device i.e. iPhone and/or iPad?
It is currently not possible to change the format of screenshots from PNG, however there are many ways to change image using a PC.

As for your second question, this is only possible by jailbreaking your iOS device and installing SSCapture.