Images not being uploaded

I am trying to develop a website using Pinboard theme but I am unable to upload any image files even though they are within the size/dimensions restrictions. Whenever I upload an image, I see an invalid icon.
I was trying to upload images for header as well slider but I am unable to upload any images for the site.
What do you think might be the problem? Has anyone faced such an issue before?
Link to my site : https://www.royalcpop.com/

You can see the invalid image icon on the slider position on home page.