Select all at one time the images that will be involved in getting the same transition, then Timeline Menu/Apply Default Transition.
For selecting all at one time, consider
a. As you hold down the Shift key, click on each of the images involved
b. With the mouse cursor, draw a rectangle around the involved clips to select them
c. Edit Menu/Select All (or Ctrt+A) if you want all on the Timeline selected for this.
Please consider. Any questions or need clarification on the Adobe, please do not hesitate to ask.
Add on....for selecting the Default Transition...Transitions Tab, just right click the transition to be set as the default, select Set Selected As Default Transition from the pop up tab.