You can add a logo to your Recruit account by going to Setup-->Job Boards -->Edit Settings [for Public Job Board]:
Then add a logo file here:
You can also add a Header Return URL which will link back to your website by adding in your website address here:
Once you save your changes, confirm that your logo size looks good to you via the link to your public job board found here:
or by clicking here:
Use a photo editing tool such as Paint in Windows to resize your logo. Typically a 300 x 300 or 350 x 75 image file looks best.