You can try the following code in the widget template:
<%@ Register TagPrefix="thm" Namespace="Telerik.Sitefinity.Samples" Assembly="ThumbnailSelectorField" %>
The reference is related to the thumbnail selector, that is created by following the blogpost by Slavo. If you have another namespace please make sure to change the reference accordingly.
Thumbnail code -
Value='<%# Eval("Thumbnail")%>' > </
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