Needing Some Help With A Landing Page!

Alright, so I have a designed image that is essentially a landing page that talks about my upcoming app - there’s no buttons or email signup or anything, it’s just a longated picture explains the application prior to its launch. I have one made for when viewing on mobile, and then one made for when viewing on desktop. My question is...when starting a hosting service, how do I go about uploading both pictures and making it so that it recognizes when the person viewing the website is using mobile or desktop? Also, do I need to use WordPress, or is a different alternative better for simply putting a picture on the domain as is?

What you’re after is media queries. Have a look at these to learn what they are and how to use them: https://css-tricks.com/css-media-queries/

https://www.smashingmagazine.com/2018/02/media-queries-responsive-design-2018/

https://developer.mozilla.org/en-US/docs/Web/CSS/Media_Queries/Using_media_queries