As I show you in my book on Amazon, if you select the entire set of clips on your timeline, you can simply right-click on the group and select Timeline/Apply Default Transition -- the Default Transition being, by default, the Cross Dissolve.
Though there are actually four different ways to build a slideshow in Premiere Elements, and each has its own way of adding transitions and pans & zooms.
Lasso them with your mouse or shift-click each one.
Or simply press Ctrl+a (Cmd+a on a Mac) to select everything on the timeline.
If I'm seeing your timeline image correctly then you have space between the Jpegs? You cannot apply cross fades across a gap between images.
You would probably want to apply a fade in and fade out to the images.
So select all the jpeg's and right click on one of them. Then select "Fade" and choose Fade in and Out. Or chose one of the other available options.