Several methods...
Under OPTIONS | MEDIA tab set the default still image duration to be the total audio length divided by the number of clips. Then load the clips into the bin and drop them onto the timeline.
Or select all the clips that are on the timeline, right-click and CHANGE DURATION. In the duration box (shown at the right here), enter the the number that was the result of the division.

Or select all the image clips and drag the last clip border in short increments to the right until it matches up with the end of the song. If you make a mistake, press <ctrl-Z> to backtrack. (EDIT | UNDO)
Using any method, If a slight adjustment is needed at the end, hold down <shift> and drag the final clip so it matches the audio end.