Your logo can been added as both an image within the main header of your forum and as a 'favicon' which will appear within the browser address bar, tab or or next to the site name in bookmarks.
Add a favicon using the same logo image
Convert your logo into ICO format, which is a standard used by web browsers and other programs for icon images. There are a number of free online converters that will do this, such as favicon-generator.org. This site allows you to create a number of different types of icon files, but in this case we will only need a 16 x 16 version which is standard for the web. Make this selection, and then upload your logo. The site will convert this into a favicon that you can download and save.
Return to your forum settings and upload your new favicon, and then save settings again.
Your logo will now be featured on your forum, both on the page and within the browser.