Removes one thumbnail or a group of thumbnails from the WebThumbnailViewer control.
firstIndex
The 0-based index of the first thumbnail to remove.
lastIndex
The 0-based index of the last thumbnail to remove. Must be greater than or equal to firstIndex. You can pass -1 to remove from firstIndex to the last thumbnail index in the WebThumbnailViewer control.
Refer to Insert(string,int32,int32,int32)