That's a fairly large picture that you're trying to put in your sig (roughly, 400px × 300px, and around 40KB), so that may be what is holding you up:
the signature image upload menu wrote: The maximum size of your custom image is 500 by 100 pixels or 19.5 KB (whichever is smaller).
Meaning, even if you are hosting off of Photobucket (or other sites of the like), if the image is still too big (dimension wise), it won't upload.
And often times, IE will not load the "error" screen of CAA saying that your image is too large, and instead just refuse to work (I know this from using IE back when I first joined CAA). Resize the pic down to under the requirements if you can and try uploading the pic again. If that doesn't work, then it may be something more technical.