resize homepage picture

I am a wordpress newbie (hopefully, I’m asking the right question and using the correct terminology.) I installed Themetrust “Clean” on to wordpress. I tried to install the demo content so that I could use the demo pages and post as a template. I am currently having alignment issues on my homepage. (I believe it’s because my draft picture is not the proper size) I would like to have 4 columns perfectly aligned under the homepage demo picture. (www.sheaedwards.com)

Currently I have a draft header photo there. Stay on homepage and click over to the 4th button under the picture. There you will find the demo (yellow) picture with the correct size photo that aligns with the 4 columns beneath it.

For some reason, I can’t find that picture on the backend (wp dashboard) so that I can locate the size and upload another picture in it’s place. Can someone please help me? Thank you in advance!