I have some important documents in pdf, as well as some pictures in jpg and s.g format. I backed them up using backintime. Now if I just copy that backup to some thumbdrive and do a fresh install. Will I be able to get my files back???

My guess would be no. All the documentation I've read about backintime is that it takes a snapshot of your system. So basically it's just a file with pointers of data on your disk. As you make changes to your disk the snapshot grows because the pointers turn into data which was changed using a diff. So if you take away the disk then all of the pointers to files which you have not changed will no longer be there so the backintime snapshot will just be a useless file.

That is if they're using the term snapshot correctly. That's the difference between an actual backup and a snapshot. One is a full copy of data (backup) and the other is a file of pointers to data with only differences stored (snapshot).

I'm currently viewing the source to verify though.

The question of the OP is to be answered "yes, you can".
BackInTime uses hardlinks to "de-duplicate" unchanged data between the "snapshots" (be the term right or wrong...).
Any snapshot folder in itself is a complete copy of the backupped directory structure.
But make sure to stay on linux filesystems (not a default-formatted USB thumdrive or such) and make sure you use the right parameters to the copy command...