GIMP auto adjusts an image for you when you initially change either width or height. When you go to change the size of the other mesurement, it will change it too a number so that your picture comes out normal looking.
You could hold CTRL and drag the scale, that woudl work well for you. Either that, or switch to %age when resizin,g and make sure you have the same %age on X and Y. Basically, don't stretch your image to fit the canvas, just make it small/big enough to stand out.
I think the reason was because you need to explain more on how it's good. Name the specific good and bad qualities of it. As for your sig, it made me LOL very hard. Wanna know why? Spoiler The soldier looks like he just pissed himself The text was okay but it could have been better because the word seems to blend in with the background a little too much. Don't choose pics where the person (or anything you are going to use) just stands there. For beginners such as myself, I find it hard to make a sig with just a face at a steady position. /