In SharePoint, different types of list can have different icons:
That’s fine, but SharePoint 2010 adds a second icon – one that’s used in the Silverlight control for creating new Sites/Lists:
I wanted to specify that for a list definition I was upgrading from a 2007 version. But there doesn’t seem to be a property to set the name/url for this file.
In SharePoint 2007 the icons were usually .GIF files. In SharePoint 2010, those still seem to be in the IMAGES folder in 14-Hive, but there are .PNGs of the same images too. There are also the larger PNGs used by the SilverLight control. The small sized ones all seem to be called ‘ITxxx.PNG’, and the larger ‘LTxxx.PNG’:
So, I tried changing the Image property of my list definition to use a file with a .png extension (e.g. ITandy.png )
I converted my GIF to PNG format, and I created a 64×64 pixel PNG for the SilverLight control. I called this new PNG file LTandy.png – and low, the SilverLight control picked it up. It seems that the Silverlight dialog replaces the IT at the start of the image file name with an LT when looking for an icon – but only for lists where the smaller icon also has the .png extention.
Therefore, I’d recommend always using PNGs for icons in SharePoint now.
Andy Burns' Blog 
Great post, this actually helped me quite a bit.
Works perfectly! Thanks for sharing!
I was only only to change the icon as it appears in the Silverlight control. The icon on the View All Site Page remains the same. I modified the ListTemplate’s Image attribute from LTxxxx.png to ITxxxx.png. There was no difference. Am I doing something wrong?
I think that the ListTemplate should still refer to the smaller ITxxx.png. If I remember correctly, the SilverLight dialog automatically looks for the LTxxx.png if the list template contains a reference to ITxxx.png.