I think I understand how picture thumbnails are generated/cached/hashed:
It calculates crc32 from the url in format of:
URLSAFE means that all non letters&numbers ![a-zA-Z0-9] are converted to corresponding hex codes.
So my original example would become:
I haven't found yet where exactly in code this is done, but I traced it now to (TextureCache.cpp) CTextureCache::CheckAndCacheImage()
The most helpful hint was to go check the Textures.db database, where you can see full url which was used for hashing.
Now I only need to find what does the image hash stand for in the texture table and then I think I will be able to develop batch image thumbnail generator.