parameter
You can only play with opacity on each picture.
Example of code:
<defs>
<filter id="MyFilter" filterUnits="userSpaceOnUse" x="0" y="0" width="400" height="400">
<feImage xlink:href='fondu3.jpg' result='pict1'/>
<feImage xlink:href='bateau.jpg' result='pict2'/>
<feMerge>
<feMergeNode in="pict1"/>
<feMergeNode in="pict2"/>
</feMerge>
</filter>
</defs>
<use filter="url(#MyFilter)" x='0' y='0'/>

Figure 1: feMerge with two JPEG pictures
Tool: you can choose your pictures and their opacity
FrontPage feColorMatrix feComponentTransfer feSpecularLightning feDiffuseLightning feSpotLight feDistantLight fePointLight feFlood feImage feTurbulence feTile feMerge feBlend feComposite feGaussianBlur feMorphology feConvolveMatrix feDisplacementMap feOffset