We recently had a consultant build a web part for us that displayed thumbnails from images in a document library. In digging around in the code they gave us I discovered that they used a page built into Sharepoint called AssetUploader.aspx (in 12\TEMPLATE\LAYOUTS\AssetUploader.aspx, available off any web at _layouts\AssetUploader.aspx). This page is a rather handy. It’s specifically designed to take a URL of an available image, resize it, and spit out a thumbnail. You can use it in any page or web part in Sharepoint by creating an IMG tag like this:
<img src=”http://www.blogger.com/_layouts/AssetUploader.aspx?Size=Medium&ImageURL=/mysitecollection/mysite/MyImagesDocumentLibrary/IMG1234.jpg“ alt=”My Image” />
If you use a size of ‘Small’, it will make the largest dimension of the image 70 pixels, and if you use anything other than small (Medium, Large, etc.), it will make the largest dimension of the image 140 pixels.
It worked wonderfully in our test environment, but once we brought it into production, it broke. All we saw were little generic white document icons - no thumbnails. Yikes!