I think a lot of people spend too much time tinkering with their ad images.
Learn how to touch up your images, frame the part of the image you want, crop, and resize to the exact dimensions required for your ad upload.
This can easily be done under 30 seconds, and if you do it a dozen times you will be able to do it under 15 seconds. I timed myself once and I did it in 11 seconds. How fast can you do one?
Finding the images that convert should be the more time consuming task!