To add a logo you must place the logo on your server in a shared drive that ALL users have access to. It cannot be placed in a local drive, such as C: or D: There is usually an Evosus shared drive which should work. Basically, everyone needs to be able to access whatever drive the logo is saved in. Logo should be a 300dpi jpg file.  That will allow the file to be resized in Evosus without losing resolution. 

Once the logo is saved, go to Admin>System>Stores>Stores>Logos. Browse to the folder where the logo is located and select the logo. Click OK to save.