SSL 'mixed content; error on IndyCrowd.scot

Hi
I've had SSL set up for indycrowd.scot, but Woocommerce shows it as not being set up.
My techs advise me it is because some content is stored as http not https, specifically this image:
indycrowd.scot/wp-content/uploads/2018/07/block3-403x290.jpg
This results in a 'MIXED CONTENT' error, causing fluctuation in port 80 and 443, is what they advise.
They say they can run a search and replace to fix the issue, but my concern/question is about how this happens - How is it possible to upload an image to an SSL operating site, and have it be stored in a non-SSL way??
This would mean it could happen again right?
Thanks, Neil.