Why is my banner so large? Please help.

I am starting to build my second site on Squarespace and am using the "Jones" template.

I do everything including resizing my image. Why is it so enlarged? How do I make it appear normal?

The dimensions were changed to 286 x 1000 pixels wide and as you can see it is too large.

Also attached is the image that I want to have as the banner.

I am of familiar with building a site from scratch. This is the first time with a template.

Any help is appreciated. :)