first click on the tab at the top opf any signs101 screen that says "User CP" & once there, you will notice a number of links in the upper left corner. the top one is "edit signature"
(see first pic below)
Click on that, & you will see both a preview of what you currently have, & a display whatever you may have typed for a signature & the html, or vB code that was created for you when you first established your signature. (like the image and email tags shown in the screenshot of my signature. My guess is this is where you could delete the old image.
(see second pic below)
Then, at the bottom of that dialog box, you will see another box where you can upload another image, &/or bring up a preview of this, or any other typed changes you may have made in the upper box.