How can I open or convert JTIP (jpeg tiled image pyramid) files?

Question

I've found some old photos (.jpg) from 2005 on a harddrive and can't find a way to view them. Using droid, I found out that these seem to be JTIP (jpeg tiled image pyramid)-files. The puid of this format is fmt/149 according to nationalarchives, if that helps someone.

Does anyone know of software which can open or convert these JTIP files?

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The following is probably not important, but maybe I've made a mistake along the way and someone doesn't mind reading my odyssey, so here's what lead me to believing I need something to view jtips:

After I first found the pictures, I tried opening them with various programs (Gimp, Photoshop, Native Windows Photoviewer, QuickTime 7) and changing the file extension to various others, whereas none of which could open any of them.

My first thought was some kind of data corruption, so I scavenged for recovery tools and jpg repair tools. One said that, when viewed in a hex editor, a jpg is only repairable if its contents seem random and aren't overwritten with FF or 00, which is understably enough; when viewed in hex, the files in question looked different to other 'healthy' files I've looked at though: The problematic files start whith seemingly random rows immediately, whereas healthy files have a lot of 00 mixed in for the first few rows and only become random afterwards. Also, they end on ~20 rows of only 00, whereas the healthy files naturally don't.

Most of the tools gave up with the pictures or asked for a non-corrupted sample file from the same source, which I don't have. The tool JpegRepair from DiskTuna however showed something interesting: All the files have an entropy of 8,0 bits/byte, which leads the program to the conclusion, that the files would be encrypted. I have no idea why these files should be encrypted somehow, also there were other pictures in sibling folders that worked just fine.

Having no other lead, I started (unsuccessfully) searching for ways to brute-force a with an unknown method encrypted jpg, and found droid along the way which was mentioned in one example.

Droid then pointed me to the jpgs being jtips, which would be an explanation as to why I can't open them. From then on, I only found the explanation of what a jtip is by nationalarchives, and one unanswered question on stackoverflow which links jtips to SPIFF files, about which I couldn't find anything useful either.