Search Style 3 Image background

I am trying to change the background pic in the search box on the homepage.

I have read to change the image bg1.png, but I am using a full colour photo and saving it as a .png will make the file size huge.

I tried to load the CSS in custom CSS plugin, but the template overrides the custom css.

Is there a way to override the css? and is there a way to change this image in the backend?

thanks in advance!