What's the explanation for "thumb/5/54"
in the example above?
Are there rules for conversion?
It's based on the MD5 hash of the image name. The MD5 Hash of
"150px-Flag_de-berlin_civil_300px.png" is 5490af54dffb956a9455f50a1f954eee.
The first folder is the first character, and the sub-folder is the first 2
characters.