10,000 pixels is massive for a single image. They must be auto compressing it to make sure it's not super heavy and negatively affecting user experience (waiting for the image to load).
I recommend breaking the image up into multiple images. Also if the image has text in it, that should be placed in code, not an image. Text renders instantly on page and require no load time, and will always have perfect clarity. Images often don't render text very well, and are heavy objects so affect your load speed.
Founder of
Speed Boostr (Shopify optimization experts, theme customization, custom app development).
Creator of
Shopify Analyzer (first performance analysis tool for Shopify sites, free for the Shopify community).
Creator of
Order Automator (auto tag orders and customers + auto fulfill orders + email notifications).
Creator of
Theme Scientist (A/B testing app) and
Tip Jar (add a tip button to your store).
