+xI tried to set inputdevice=mouse in <defaults>, but unfortunately that didn't work. The problem is still there. Or do I have to change something else?
There's still something in your script that requires an input button (regardless of mouse or keyboard input), namely the instruction page.
Thus space continues to be reserved at the bottom of the screen for this purpose. If you strip that page out, it'll be gone.
<defaults>
/screencolor = (255, 255, 255)
/fontstyle = ("Arial", 2%)
/ posttrialpause = 500
/ canvasposition = (50%, 50%)
/ inputdevice = mouse
</defaults>
<expt>
/ blocks = [1= practice]
</expt>
<block practice>
/ trials = [1-2=list.trialselectiontest]
</block>
<list trialselectiontest>
/ items = (trial.pic1, trial.pic2)
/ selectionmode = random
/ itemprobabilities = (0.5, 0.5)
/ poolsize = 2
/ replace = false
</list>
<trial pic1>
/ inputdevice = mouse
/ validresponse = (lbuttondown)
/ stimulustimes = [0=fixation; 500=erasefixation, pic_1]
/ beginresponsetime = 0
/ response = timeout (3500)
</trial>
<trial pic2>
/ inputdevice = mouse
/ validresponse = (lbuttondown)
/ stimulustimes = [0=fixation; 500=erasefixation, pic_2]
/ beginresponsetime = 0
/ response = timeout (3500)
</trial>
************************** stimuli *****************************
<picture pic_1>
/ items = ("pic1.png")
/ position = (50%, 50%)
/ size = (70%, 70%)
/ erase = false
</picture>
<picture pic_2>
/ items = ("pic2.png")
/ position = (50%, 50%)
/ size = (70%, 70%)
/ erase = false
</picture>
<text fixation>
/ items = ("m3")
/fontstyle = ("Arial", 10%)
/ position = (50%, 50%)
</text>
<text erasefixation>
/ items = ("+")
/ txcolor = (255, 255, 255)
/ size = (100%, 100%)
/ position = (50%, 50%)
</text>