MacOS – How to use one of the documents preview icon as the parent folder’s icon in OS X

As mankoff said, the large size is probably a resource fork (with things like previews). To confirm this, use the Terminal command ls -l@ /Volumes/Database/WebFolder/imgs/confirm.gif -- you'll probably see something like this:

$ ls -l@ /Volumes/Database/WebFolder/imgs/confirm.gif
-rw-r--r--@ 1 www  www  539 Nov 18 09:39 /Volumes/Database/WebFolder/imgs/confirm.gif
        com.apple.FinderInfo     32 
        com.apple.ResourceFork  40748 

If it doesn't show a 40kb resource fork, report back so we'll all have a better idea what's going on.

As long as the image is just for web use, a big resource fork isn't really an issue, since the web server will only serve the data fork (which is just 539 bytes long). If want to compact the file anyway, use the command xattr -d com.apple.ResourceFork /Volumes/Database/WebFolder/imgs/confirm.gif to remove the fork.

This icon is shown on files that have been downloaded from the Internet and not used yet.

The first time you open the application, you will see a dialog warning you that you downloaded the application from the Internet and asking if you are sure you want to run it.
Say yes and run the app, and the icon should soon be changed back to what it should be (you may need to log out and back in).

To answer your question in general:

To change an app's icon, you first need to copy the new icon you want to use to the clipboard.

1. Open the image file you want in Preview (found in your Applications folder),
   select all (⌘+A), and copy (⌘+C).

2. Find the app whose icon you want to change. Select it in Finder and choose File->Get Info (⌘+I).
   

3. Select the small preview icon in the top left of the Get Info window, and paste (⌘+V).