What yuo need to do is to first open the picture in an image editor
and resize it. While you're at it you can also crop the image etc if
need be.
There's plenty of software out there for free. I use Adobe Photoshop,
but that's not free software. Back in the day I used to use Paint Shop Pro
by JASC software.
The image you'd uploaded was huge - so I took that off. Here's the
same picture - resized.