Issue with Dynamic images not always showing

I am having an issue with images not always showing on a dynamic gallery,
when using the development tools in google chrome the error is reported as
{ Failed to load resource the server responded with a status 403()},
The issue is not always happening on the same image if I copy and paste the path into a browse tab the image appears.
The website is https://www.kokowai-artist.nz, I have noticed that the browse is not always detecting the correct file type for the image which is a Jpeg. This is happening across a number of browsers.
Not sure what is going on has anyone any ideas and can point me in a direction to look at.
Have been wondering if could be related to having just implementing a trusted server certificate.