If width and height are not specified the default is used. To handle the case where we have less images than we need to fill out this row the <FixedContent> is used in place of the Image tag to fill out the row.
<Image ThumbWidth="100" ThumbHeight="100">
some HTML content referencing the {ImageThumbFileName} variable name.
<FixedContent>
An example of fixed content that's used when we have no more
images to display on this row
</FixedContent>
</Image>