saggi What about WRAP columns?
That doesn't quite give the layout I'm looking for, because it places the images at the left edge of their respective columns. So I added a space to the left of the first image and another one to the right of the second image:
<WRAP group>
<WRAP half column>
{{ :wiki:image1.png?400|Image 1}}
</WRAP>
<WRAP half column>
{{:wiki:image2.jpg?400 |Image 2}}
</WRAP>
</WRAP>
That pushes the images to the middle with a nice margin between them. But if the images don't have the same width (as mine do), the combined image is not horizontally centered anymore.