Lewisite

Alright, so I am about to lose my mind to say the least. I consider myself pretty tech savy, cannot get this to work at all though. I am simply trying to stretch an image or figure out a way to resize it to use as my background. I download a picture to my phone, and the little window you drag simply does not fill up the image. It will hit a barrier and you cannot drag it anymore, no stretching or no nothing, it simply stops.

What the heck am I doing wrong? Is there another way to resize the photo and then send it to me phone or what do I need to do? I am about to go batty.

Thanks in advance!