You put us at a disadvantage not know what version of Premiere Elements you are using and on what computer operating system it is running.
In view of that, I will offer a generalized answer.
Select all your photos all at one time, go to Timeline Menu/Apply Default Transition.
Beforehand, you can set the Default Transition from a lineup of transition in the Transition display area.
Details are needed from you so that the response can be customized for your workflow.