feMerge filter

parameter

in: name of RGBA raster

You can only use opacity on each picture.

Tool: you can choose your pictures and their opacity

Example of code:

<defs>
<filter id="MyFilter" filterUnits="userSpaceOnUse" x="0" y="0" width="400" height="400">
<feImage xlink:href='#MyImage1' result='pict1'/>
<feImage xlink:href='#MyImage2' result='pict2'/>
<feMerge>
<feMergeNode in="pict1"/>
<feMergeNode in="pict2"/>
</feMerge>
</filter>
<image id="MyImage1" opacity="0.4" width='400' height='400' xlink:href='fondu3.jpg'/>
<image id="MyImage2" opacity="0.4" width='400' height='400' xlink:href='bateau.jpg'/>
</defs>
<use filter="url(#MyFilter)" x='0' y='0'/>