To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
How do I upload a website button to be displayed on my userpage?
You can upload your website button by going to your Edit Profile page, then by clicking on the User Page Display tab. Once there in the left hand column there will be a section entitled 'Website Button'. From here click the Browse button and select the file you wish to upload from your computer. Once you have a file selected, proceed to the website URL field and enter the desired website's address. A URL must be provided for your button to be displayed. Once done click on 'Update'.
Note: You must make sure that the website button dimensions are 88 pixels in width and 31 pixels in height. The image file type must be GIF and it must not be more than 15kb in filesize.