Feedback Display Image on Timer?

Hello! I am new to unity and am still learning C#. I am trying to display a random image from about 18 images every 30-45 seconds. I have a timer set up that counts in minutes and seconds displayed on screen as well as a small array of three of my images for testing. I want a random picture to show up, stay for about 10 seconds, then disappear, with a new image showing up about 30 seconds later and repeating this process until all 18 images have been shown. This is my code so far, but right now once seconds passes 10 it changes a lot, and doesn't stay. Any ideas on how to fix this would be great, thanks!