When I link to my page an old event image pops up (On iMessage / Facebook / etc...)

I could use some help. 

We use our site for our annual fundraising dinner. I have updated our information on our main page but there seems to be a "hidden page" that is the image that shows up when i share the link in iMessage / Facebook. I cant even find that page / image that is being linked (Its from our event a few years ago) 

Does anyone know how to change that image that shows up when we send a link to our site?