Dear  @frican
The image resize logic you will be find at /root/app/Http/Controllers/CampaignsController.php at line number 258
Find this code 
$resized = Image::make($image)->resize(300, null, function ($constraint) {
                $constraint->aspectRatio();
            });
and replace null to a specific height, i’ve given you the example code
$resized = Image::make($image)->resize(300, 250, function ($constraint) {
                $constraint->aspectRatio();
            });
Just copy and replace the above code to there, the image will be resized to this size.
Beware: it will be stretched your image if it not well ratio.
Best regards