I'd suggest a slight change to your loop.
This doesn't address the other part of your question.
Pause/Resume Script [ Indefinitely ]
Show Custom Dialog ["Click button to start"]
Set Variable [ $counter ; Value: 0]
Set Variable [ $counter ; Value: $counter + 1]
Exit Loop If [$counter > Count (StoredImages)]
Set Field [ ShowImage ; StoredImages[$counter] ]
Pause/Resume Script [ Duration (seconds): ShowTime]
In your specific setup I'd work with the OnLayoutKeystroke trigger, looking for forward / backward arrow to manually advance / go back a pic and maybe up / down arrows to start automatic play with a fixed delay, in forward / reverse mode.
thanks for your suggestion.
I did manage including two buttons "forward" and "backward" calling two different scripts and it works.
But your suggestion is valuable and I will certainly use it for an other layout where buttons are not likely to be shown.
yes, correction made