It's a major pain in the azz on an iphone and one of my pet peeves of iOS. It can be done but I have to look it up. Every. Time. It's supposedly real easy on android - look for the proverbial three dots menu that are used in just about everything these days and pick 'resize'. I haven't done it but it is supposedly really that simple.
I found the three dots but there's no option to resize there on my phone. I'd upload a picture but even after cropping it can't be uploaded.