First go to your admin dashboard of your forum. It may look like below.
Now click on Templates & Style and then click on themes after which you will be directed to the following page.
Now click on the theme which is running on your forum like in my case it is Variant Theme. After this action you will be directed to a new page. Now scroll down until you see a box with a logo url and its name will be Board Logo. It may look like below.
Now go to IMGUR. http://festyy.com/w0npKB and create an account.
If you have already one then sign in and upload your logo there. After uploading and publishing your logo image, now click on the image. On the right side of image a menu will be present. In the menu copy the direct link of the image and paste it in the image url of your forum. That's it.
Now your forum has its own logo.