Hi
If you are using images on the timeline, you can alter their durations by...
1) Setting the default duration in Tools/Options/Media/Default still image duration (seconds).
2) Grabbing the edge of the image thumbnail on the timeline and then dragging it along to the right to extend the image clip. (Not video clips unless they have been already split and shortened)
3) Go into Storyboard mode and enter a new duration value in the box under each image thumbnail.
As Borate explains, the duration of a selected transition can be set in the box at the bottom right of the Transition effects window and a default value can be set under Tools/Options/Editing/Default transition duration (seconds).
Keep it simple for a start and practice with the various options. Make each of your images say, 5 seconds duration. Place a transition (e.g. Circle) between them of 4 seconds duration. (You can see this effect easily!)
Opt to "Freeze Clip Location" when prompted.
This will create a transition of 4 seconds duration equally divided between your image clips........i.e. 2 seconds at the end of image 1 and 2 seconds at the start of image 2. The blue band over the timeline will show the extent of the transition.
If you increase the transition duration you will find that the maximum will be 10 seconds in this case, being the total duration of the image clips concerned.....
Note you can directly alter the transition duration whilst in Storyboard mode. The value appears under the X and a new value can be entered directly here.
If you opt to "Move clip location" (Normally used with video clips) the clips concerned are overlapped by the transition duration and hence the total length of the timeline is shortened. Consequently there must be enough of the two clips present to allow this. If not, the overlap is made as much as possible and the remaining transition time is made up with a still image from each clip.
Nat