Select Different Fallback Image on Mobile & Desktop Views

Hi, 

I'm pretty new to Squarespace so this has had me stumped for the past couple of hours and I can't find any similar solutions on the Internet that I could successfully adapt. 

I've just launched my new website (www.csvideo.co.uk) and I am using a video from Vimeo as the video background. When I load my site it takes a second or so to start the video and so shows a black background. Not a problem until you get to mobile where the lack of anything and a solid section of black looks weird. 

I thought I'd solved this by uploading a mobile fall back image that suits my website when viewed on a mobile device but then this image is shown while the video loads on the desktop and looks completely out of place! Aaarrgh! 

Temporarily I've uploaded the first frame of the video so it looks okay on desktop but the image doesnt really suit the static view on mobile so I was wondering is there a way I could tell SS to only select a certain image when its viewed on desktop or mobile? That way I could use a suitable image on both version. 

Thanks! 

Same problem here.  Really need help with this!

If you want to hide fallback image on desktop, please share site url, we can give the solution.