I would suggest that you contact httpix.com support and ask them about this. I can explain what happens here, and why the image there does not show up here.
1. You need to point to plain img URL. A gallery page is html content, not an image. EMBED IMAGE tool will extract the image url from most popular gallery sites for you.
This seems to be fine with your embed code.
2. image must be allowed to be embedded in other sites. Hotlinking or firewall, whatever the reason. How that is set up and solved is entirely up to server admin.
The problem is most likely this.
I can gladly respond to all accusations which suggest the problem is in POTN, but you'll have to present specific test cases and ask specific questions, please.
If you feel overwhelmed about all this, don't worry. Setting up a server and handling all problems is not something you can learn in few hours or days. It is pretty expert stuff and needs a lot of experience. If you want to play with servers I'd suggest installing the server software you use in the paid server to a local Virtualbox instance, that way you'll get a sandbox which can show you many things and is recoverable. Virtualbox is not easy either, nothing with servers is. I have a procedure to set up a POTN server manually from clean install of RHEL, and it takes two hours and has 500 lines of complex CLI commands. Making that procedure solid and testing it took a month.
Don't expect to master this in short time. One big problem with all this is that you have to know the right terms and understand protocols, networking basics, http, etc. before you can ask the right questions, not to mention understanding the answers. That is how it goes with Linux, sadly.