20.12.2020
In your Cloudflare account select your site and go to the section Security > WAF. Click the button Create a Firewall rule.


OptiPic Allow (you may use a name of your choice)
If the IP matches the list of IP addresses OptiPic:
5.9.115.175.9.117.98134.195.197.16965.21.79.243185.146.159.191146.88.24.2095.216.238.75
Allow
Save the new rule!
Cloudflare official Documentation for creating Firewall Rules.
In your Cloudflare account select your site and go to the section Security > WAF and go to the tab Tools.
For each IP address from OptiPic (see above in "Step 1") add a rule to allow access.
IP address from the list above
Allow
This website
OptiPic Allow (you may use a name of your choice)

In your Cloudflare account go to section Firewall > Firewall Rules of your site and click Create a Firewall rule.

OptiPic Allow (you may set the name as you wish)
If the IP matches the list of IP addresses OptiPic:
5.9.115.175.9.117.98134.195.197.16965.21.79.243185.146.159.191146.88.24.20
Allow
Save the new rule!
Rule settings screenshot:

Cloudflare official Documentation for creating Firewall Rules.