First thing you need to do. Back everything up. Trust me, I've been around the block for awhile, stop everything you're doing and back it all up. Backup your WordPress files on your hosting account and backup your database.
You can use the database plugins: WP db Backup. [spam ink removed]
Next you'll want to backup all the files on your hosting account. You can simply download them through FTP to your computer or if you know how to zip things up on your web server, just zip them up into a file for safe keeping.
Now that everything is backed up, I noticed on Arthemia the logo is of type .png, not .jpeg. So if you can convert your .jpeg to a .png file, that would be helpful. If you like, you can link to your image here and I can convert it for you (or perhaps someone else who gets here faster).
Next take that logo.png file and upload it through FTP to the following dirctory:
You will be replacing the default logo.png file which came with Arthemia. The trick here is if you want it to snap right in, your image size should be 400px wide and 100px high.
If you cannot change to a .png (I can for you or here's an online conversion tool) then you'll need to edit your header.php file which you'll find by logging into WordPress dashboard – Appearance – Editor and edit Header (header.php) as that article shows you.