Hi
having added your images to the timeline they will all have , at the outset a duration equal to the value set in the Option/Media/Default still image duration.
If you want to alter individual image durations then simply grab the right hand edge of the image clip on the timeline and drag it to the right. This will length the image clip and it will play for longer. (Zooms and other effects are automatically adjusted accordingly)
As an alternative you can right click on the image in question on the timeline and then select Change Duration from the menu option shown
You can also switch into Storyboard mode, right click on and and change the value shown under each clip.
Nat