I brought pictures. First, pull up
imgur.
Click that.
Your pick of which to choose. In this case, I went with "Browse Your Computer".
This is assuming you've saved the image you want to your computer. Locate the image, as detailed by 1, then either double click it or press "Open", as seen by 2.
Make sure it shows the photo is selected, as I have underlined, then click "Upload Photo".
Right-click the designated link and hit "Copy". Now just go into your Signature and paste it with either right-clicking and hitting "Paste" or press Ctrl+V.