views 3: select only one value from a multivalue field
It under the "multiple field settings" in the settings of your image field. 

You should choose "Display all values in the same row", separation mark: "", display 1 item, offset 0. (translated from my own language so names might be a bit different)

UPDATE
You will need to display fields rather then the whole node though. (I don't know how you did this before)