You essentially would do something like this (using <text> elements for illustrative purposes ony; it works the same with <picture> elements):
<values>
/ condition = ""
/ youravatar = 1
/ player1avatar = 1
/ player2avatar = 1
/ player3avatar = 1
</values>
<expt same>
/ onexptbegin = [values.condition = "same"]
/ blocks = [1=myblock]
/ subjects = (1 of 2)
/ groupassignment = groupnumber
</expt>
<expt different>
/ onexptbegin = [values.condition = "different"]
/ blocks = [1=myblock]
/ subjects = (2 of 2)
/ groupassignment = groupnumber
</expt>
<block myblock>
/ trials = [1=selectavatar; 2=mytrial]
</block>
<trial selectavatar>
/ ontrialend = [if (trial.selectavatar.response=="target1") values.youravatar=1]
/ ontrialend = [if (trial.selectavatar.response=="target2") values.youravatar=2]
/ ontrialend = [if (trial.selectavatar.response=="target3") values.youravatar=3]
/ ontrialend = [if (trial.selectavatar.response=="target4") values.youravatar=4]
/ ontrialend = [if (values.condition=="same") {
values.player1avatar=values.youravatar;
values.player2avatar=values.youravatar;
values.player3avatar=values.youravatar; }
]
/ ontrialend = [if (values.condition=="different") {
list.targetitems.removeitem(values.youravatar);
values.player1avatar=list.targetitems.nextvalue;
values.player2avatar=list.targetitems.nextvalue;
values.player3avatar=list.targetitems.nextvalue; }
]
/ stimulusframes = [1=target1, target2, target3, target4]
/ inputdevice = mouse
/ validresponse = (target1,target2,target3,target4)
</trial>
<trial mytrial>
/ stimulusframes = [1=you, player1, player2, player3]
/ validresponse = (57)
</trial>
<text you>
/ items = targetoptions
/ select = values.youravatar
/ position = (25%, 25%)
/ txcolor = red
</text>
<text player1>
/ items = targetoptions
/ select = values.player1avatar
/ position = (75%, 25%)
</text>
<text player2>
/ items = targetoptions
/ select = values.player2avatar
/ position = (25%, 75%)
</text>
<text player3>
/ items = targetoptions
/ select = values.player3avatar
/ position = (75%, 75%)
</text>
<list targetitems>
/ items = (1,2,3,4)
/ selectionrate = always
</list>
<text target1>
/ items = targetoptions
/ position = (20%, 55%)
/ size = (20%, 20%)
/ select = 1
</text>
<text target2>
/ items = targetoptions
/ position = (40%, 55%)
/ size = (20%, 20%)
/ select = 2
</text>
<text target3>
/ items = targetoptions
/ position = (60%, 55%)
/ size = (20%, 20%)
/ select = 3
</text>
<text target4>
/ items = targetoptions
/ position = (80%, 55%)
/ size = (20%, 20%)
/ select = 4
</text>
<item targetoptions>
/1 = "Avatar_yellow.PNG"
/2 = "Avatar_green.PNG"
/3 = "Avatar_red.PNG"
/4 = "Avatar_blue.PNG"
</item>
Entering an odd (1,3,5,...) group number will run the "same" condition. Entering an even (2,4,6,...) group number will run the "different" condition. I also would not use a <surveypage> to have people select their avatar; using a standard <trial> is easier -- just click on the desired option.
If you prefer using a <surveypage> with <checkboxes>, see this example on how to restrict the number of selectable options:
https://www.millisecond.com/download/library/v3/selectitems/selectitems.zip