People have told me how difficult this is, but I didn't believe them until I tried it myself. I went to Link Maker, entered the book's Amazon URL, got "No matches found". I went to Link Maker 1.0, entered the ASIN and all the little URLs (once I figured out where to find the "image" URL), cover image came out about twenty times larger than it should have.
Any ideas?
Don--
Sorry you've been having problems.
First, in Link-Maker, you want to enter an ASIN, or your name or the book's title, not the whole URL. I tried both in the original Linkmaker, and both worked:

Make sure, if you are looking for an ebook, that the selection is "Kindle Store" in the drop down box.
Then, once the book you are searching for comes up in the left pane, click on it to get the link options. For your signature, you'll want to choose the first image link option.

Finally, if you are working on your signature, you may wish to use our author signature tool, found in the top menu under Authors. It automates a lot of the process.
Let me know if you have any questions!
Betsy