find . -name "*.jpg" -type f -delete

This will find all files ending with .jpg and delete them.
If you'd want to do a test run first without deleting the files, you could try the following to show you the full list:

find . -name "*.pt.srt" -type f

However .... your beloved QNAP uses the BusyBox find version, which does NOT support this "delete" option.

The proper version is supposed to come with OptWare, but on my QNAP it was nowhere to be found (look in "/opt/bin" to see if "find" and "findutils-find" can be found).

In case yours (like mine) is missing that one, do the following to install the correct find version:

ipkg update
ipkg install findutils

In the directory "/opt/bin" you'll now find the following new files:

/opt/bin/find
/opt/bin/findutils-find
/opt/bin/findutils-xargs

To use this version of find, the statement changes a little bit:

/opt/bin/find . -name "*.jpg" -type f -delete