Does seafile store synced files anywhere?

TLDR;

No you won't find your a.jpg file on the server. Your files are going to be turned into blocks of bytes.

To understand

If you take a look at this part of the documentation of data model

FS

There are two types of FS objects, SeafDir Object and Seafile Object. SeafDir Object represents a directory, and Seafile Object represents a file.

Block

A file is further divided into blocks with variable lengths. We use Content Defined Chunking algorithm to divide file into blocks. A clear overview of this algorithm can be found at http://pdos.csail.mit.edu/papers/lbfs:sosp01/lbfs.pdf. On average, a block's size is around 1MB.

So backing up files will won't be as easy as making a raw copy of the seafile drive. As mentioned by @JensV you may still achieve something along those lines using the seafile drive client.