Lisa

There's multiple ways to do it, I usually do it like this.
 
1. Go to Imgur
2. Upload your signature onto there
3. Once uploaded right-click it and press "Copy image"
4. Now on MLPForums click on your name, and go to "My Settings"
5. Click on "Signature" on the left
6. Scroll down to the "Edit Signature" area, right-click and paste your image there.
 
Hope this helps

First of all, that signature is over the maximum size of 600x100px. You aren't able to set that specific image as your signature until you donate to the website.
 
Second, if you do donate or create a smaller version, go to this site, upload the image then right click on it, copy it and paste it into your signature box.
 
Correction: I thought the maximum height was 150px. Sorry, it's actually 100px.