(Sorry @Myrra , but this thread is terribly outdated! I'm helping you out by updating it. ^_^)
These are the methods FlashD recommends for adding a signature to your profile!
*** First Method ***
Go in the UserCP and click on "Edit Signature". At the bottom of the page you will find an option which says "Upload Signature Picture" with two possibilities. The first one is to use a signature which is already somewhere on the internet and you will have to paste there it's URL address (be sure you paste in it's full address). The second possibility allows you to upload a signature from your computer. Just click on "Browse" and select the signature you want to use, but be sure the image matches the specifications set in the NOTE line. After that paste "[sigpic][/sigpic]" in the box in the "Edit Signature" option.
*** Second Method ***
Click on Gallery and select "Add Album". Wait the page to open, then put in the information about the gallery's title, it's description and gallery type and press "Save Changes". In the next window, find the option "Upload picture" and click it to open a pop-up window. In the pop-up click on "Add files" and after that on "Select files". This will open a new window where you can select the signature from your computer. Once you've selected the signature click on the "Upload files" in the "Add files" interface. This will upload the image on the site. After it's done click on "Done" at the bottom of the window. The pop-up will close and you will be back in the gallery with a new page where you will have to select the signature's name, it's description and if you want it as the album's cover. When you've entered all the info click on "Save changes" and you will be redirected to the gallery. Find the pic you uploaded and copy the "BB Code IMG" tag. After that go in the UserCP under "Edit Signature" and paste the IMG code in the box under "Edit Signature". Click on "Save Changes" and you are done.
For more FAQ about signatures, avatars, and other stuff, see this thread!