By default, glFusion ships with a text based site logo (based on your site name and slogan settings) in the header. One of the first things you will probably want to do is replace this logo with one specific to your site.
The logo editor at Command & Control → Logo gives you an easy and convenient method to upload a new logo. For best results, the outer dimensions of the logo should be 100 pixels tall and up to 500 pixels wide (when using the default CMS theme).
Your web server needs to be able to write to the public_html/images directory in order to upload logo files.
Configuration Settings that Impact Logos
There are 2 configuration settings that control the maximum width and height (in pixels) of the logo:
Configuration -> Images -> Logo Tab
Be sure to update these settings if you wish to use a larger logo.