Is it possible to present an image behind another image? I basically want to present Red or Green boxes behind pictures.

Yes. All that matters is the order you put in /stimulustimes or -frames. I.e. suppose you have

<picture a>
...
</picture>

and

<picture b>
...
</picture>

then

<trial example>
/ stimulustimes = [0=a,b; ...]
...
</trial>

will first draw picture a to the screen, then picture b, i.e. picture b is displayed on top of picture a.

Conversely,

<trial example>
/ stimulustimes = [0=b,a; ...]
...
</trial>

will draw picture a on top of picture b.