Can't change the time a picture is displayed on timeline

Hi,

I'm a pretty new user and I want to create a short video consisting of videos and pictures.

I already know a few basics and have created a timeline consisting of videos and pictures, along with transitions and a music track.

However I have a few pictures that are troubling me.
Usually when I add a picture to my timeline I can change the time the picture is displayed by clicking on the end and sliding my mouse courser.
And the standard setting seems to display each picture for about 3 or 4 seconds on the timeline when I add them (drag and drop).
But there are a few pictures that are only displayed for 2 or 3 frames! And I cannot use the mouse courser to expand the time to display.
I noticed that the properties in the media pool for these pictures say that they actually have 3 (or 2) frames even though they are pictures (JPGs shot with Pixel XL).

I have created a small album to show what I mean if it did not come across what I mean (sorry, English is not my mother tongue):
imgur(dot)com/a/0BzoT

So my question is, how can I expand these pictures to also have them displayed for a few seconds on my timeline?

Thanks in advance for your kind help!