Uploading pictures from iMac

Site URL: https://www.plannedspontaneityhiking.com

Hello,

I am a new SquareSpace user and I am trying to upload pictures into my blog post and I need receiving an error. 
 

I took the photo on my iPhone, uploaded the picture to my iMac and then try to upload it into my post, but it will not work.

However, if I edit the post using the SquareSpace app on my phone, I can insert pictures just fine.

Pictures are jpeg and about 3 mb In size. Can anyone assist?

Thank you!