Some websites accept reports of this kind of abuse and will take action against it. You don't need to know the IP address for this; just contact the website administrator. But you have tried this. Tracing the person involved is a more complicated process and I would advise you to get professional legal advice before beginning.
The process involves just as much legal action as it does technical processes, because most of the data you need is locked away on privately owned and operated servers, inaccessible without cooperation from the owners or a court order compelling them to reveal it.
You can look up the registered owner of any IP address using a whois (as in "who is") tool. The registered owner is not necessarily the person who uses the address. It might be a service provider they use, for example. Sometimes people use whois anonymising services, perhaps to stop people sending spam email to their contact address. But if you dig deeply enough you can unravel the trail by contacting whoever is listed and finding out from them who uses the address.
There is no point in tracing the IP address of the website, unless you need to find out who owns the website. To trace the offending person who posted the pictures, you need their IP address instead. To do that, you have to ask the website.
The website is likely to have a log of which IP address people used to post things on it, and at which time they did so. This is privately held by the website.
Ultimately, you want to find an IP address owned by a consumer ISP, which would imply that you have now found the address of the actual computer that was used to post the pictures. ISPs hold information about which customer of theirs was leased which IP address at which date and time. Again, this information is kept private.
Alternatively, you might find an IP address belonging to a business or other organization, implying that one of their staff posted the pictures.
Your search could be stopped short by a number of anonymising technologies that, if they were used, would make it completely impossible, on a technical level, to trace where the pictures were posted from. You may be able to trace things back to a particular computer, only to find that the owner is completely innocent on account of one of these technologies having been used.
Edit: sorry, didn't read your question quite right; have changed my answer accordingly.