> after some trials it crashes saying <unable to initialize picture probe number 0>.
You use a values entry to select the item in <picture probe>
<picture probe>
/items = stims
/select = values.probestim/position = (50%, 50%)
/size = (30%, 30%)
</picture>
What the error means is that at the point it occurs values.probestim is null or equal to zero. Neither is a valid item number and there is no such thing as a "zero-th" image file. You need to make sure that values.probestim (as well as all the other values you use for item selection) are always set to a valid item number.
For example, in <trial negativeprobe> you try to set values.probestim to an item number pulled from a <list>
<trial negativeprobe>
/inputdevice = keyboard
/ontrialbegin = [values.probestim = list.negativeprobes.nextvalue]/stimulusframes = [1 = probe, in, out]
...
</trial>
That <list>, however, does not actually hold any items / item numbers at all. Hence, values.probestim will be set to null.
<list negativeprobes>
/replace = false
/ selectionrate = always
</list>
Run this short example to see what happens:
<values>
/ probestim = 1
</values>
<block myblock>
/ trials = [1=a; 2=b]
</block>
<trial a>
/ stimulusframes = [1=mytext]
/ validresponse = (57)
</trial>
<trial b>
/ ontrialbegin = [values.probestim = list.negativeprobes.nextvalue; ]
/ stimulusframes = [1=mytext]
/ validresponse = (57)
</trial>
<text mytext>
/ items = ("values.probestim is equal to <%values.probestim%>")
</text>
<list negativeprobes>
/replace = false
/ selectionrate = always
</list>