Changing PDF Icons

I am having some trouble managing PDF file icons... I am hoping someone here can help. I am running Leopard; everything up to date.

I downloaded several books in PDF format. I now want to change the icons of each PDF file to match the front cover of said books. So I click on the file, get info, click on the icon, and paste the image. Everything seems to be fine on the get info screen; the icon on the top left changes to the respective image.

I then close get info and look at the Finder window containing the PDF files. This is where the problem begins. In cover flow mode, some of the images are simply not there, or they will appear and disappear if I resize the window, or click through the preview. The icons are all correct on the bottom file listing in this mode. Looking at the icon mode, some of the icons will have changed while others will not. I primarily use the icon or coverflow modes in finder which is why this is a problem.

I cannot detect any pattern in this behavior; some icons behave normally, most do not. All of the said images are PNG. Preview is my default PDF viewer. I am willing to do away with the "preview" function (where you can click through the PDF document within finder) in coverflow mode... it is useless anyway and I feel as if this is the actual problem. I can't seem to find a relevant set of preferences/settings, however.

Am I missing something? Any help is appreciated. Thanks.

All my pdfs including pdf books pick up the front page for the icon and for coverflow. Get Info only shows the default pdf icon.
Maybe check:
Finder > Cover flow > Show view Options > Select Show Icon Preview.
Finder > Icon Mode > Show View Options > Select Show Icon Preview.

You can try removing the icons you put on by > get info > select top icon > then use the delete key.
S.