Don't know of a way to get rid of the white border other then to create a custom icon of the image and paste it onto the file. Open the image in Preview, Command-A to select the entire image, Command-C to copy it, select the Image File in the Finder and press Command-I to open an Info Window to it. Click on the little icon in the upper left corner of the window, a little keyboard focus hallow will appear around it. Press Command-V to paste the image as a custom icon onto the file. The custom icon should not have a white border.
Note: There is a bug where after pasting a custom icon, it will not show up in regular Finder windows or the Desktop. But it will show up in the Info Window where you pasted it. Don't worry. Quit/relaunch the Finder or log out and back in to see your new custom icons.
Ravenmoon wrote:
But please, please, if you MUST 'fix' things for me, give me the ability to opt out!
Open a window in the Finder, press Command-J to open the View Options window. Uncheck the "Show Preview Icons" checkbox. Poof, the thumbnails are gone (custom icons remain btw). Switch to each view style, unchecking the "Show Preview Icons" in each. Hope that helps.