This would work:
multirow create thumbnails item
multirow append thumbnails {<img src="image1">}
multirow append thumbnails {<img src="image2">}
multirow append thumbnails "Image 1 text"
multirow append thumbnails "Image 2 text"
multirow append thumbnails {<img src="image3">}
multirow append thumbnails {<img src="image4">}
multirow append thumbnails "Image 3 text"
multirow append thumbnails "Image 4 text"
Then:
<table>
<grid name=thumbnails cols=2 orientation=horizontal>
<if @thumbnails.col@ eq 1>
<tr valign=top>
</if>
<if @thumbnails.rownum@ le @thumbnails:rowcount@>
<td>@thumbnails.item@</td>
</if>
<if @thumbnails.col@ eq 2>
</tr>
</if>
</grid>
</table>
Note the weird ordering in the creation of the multirow. Or, you could remove the "orientation=horizontal" from the <grid> tag and order the multirow like this (might be weirder still):
multirow append thumbnails {<img src="image1">}
multirow append thumbnails "Image 1 text"
multirow append thumbnails {<img src="image3">}
multirow append thumbnails "Image 3 text"
multirow append thumbnails {<img src="image2">}
multirow append thumbnails "Image 2 text"
multirow append thumbnails {<img src="image4">}
multirow append thumbnails "Image 4 text"
Maybe there's a better way, but the above should definitely work. Good luck...