The current iteration index is available via the status
attribute of the <s:iterator>
tag. In your case is #incr.index
. If you want to display that index
<s:iterator value="images" status="incr">
<s:property value="%{#incr.index}"/>
then scriplet could be changed to OGNL expression
<s:property value='#attr.imageCaptionsString.split(",")[%{#incr.index}]'/>