Need an image size / format changing on a website using the WordPress CouponXL theme

I have had a website built using the CouponXL theme on WordPress. The website domain is TakeTheDeal.co.uk and it is a live site.

However, my developer was unable to amend the image size used on one of the pages and this is what I need fixing!

To view the problem;

1. Got to TakeTheDeal.co.uk

2. Click on any one of the live vouchers

3. This brings up what I call the voucher page. The featured image here is the image I'm concerned with. It is stretched and blurry.

I would like this image to be made smaller as there is no need for it to take up such a large amount of space.

So, that's the challenge! Ideally I would like someone who is familiar with the Coupon XL theme but if not then as long as you are comeptent in WordPress development.

It is also fairly urgent.

May be more small 'tidying' jobs to do once this is completed so looking to build along term relationship with someone who can help bring on this site.