It is always better to store the graphic in external server because:
1) The graphic files can be complex and large since it is hosted on external webserver.
2) The graphic file is loaded from the external web server to the browser on the PC. It is not from the PLC and therefore the graphic can load much faster and it also does not take up any of the PLC's CPU time to serve up a large graphic file.
Having said that, if need be you can store the JPG on the Nano-10 webserver if they are named properly in the file name format supported by the PLC (0.xxx....9.xxx, A.xxx to T.xxx etc). Also you need to calculate the file space it occupies.
Note that Nano-10 built-in server space only has 60K bytes so it is not possible to store complex graphic files. You can add a FRAM-RTC-256 to add another 256K bytes of file space if you prefer to store graphics inside the PLC.