Step 1 - Transition
If you mean something like flipping cards transition - Video Sequencer doesn't have this kind of transitions. You would have to render this kind of transition as black and white animation (or image sequence) and use as a mask in video editor.
If you need to keep 3D effect of rotated parts of images you will have to create material for Front/Back side and in 3D editor.
... with Animation Nodes
To create transition like this you can try to use Animation Nodes add-on to create a node tree like here:
Node-tree for 2.92
Step by step tutorial is a bit outdated, but it is well explaned.
... with Constraints
Another option can be to use Object Constraints with a help of BlenderSushi script for Blender 2.79 to handle tedious tasks.
- Add a Grid mesh like 6x4 segments
- in Edit mode select all search for Split Edges
- Add another Grid mesh, but with -1 segment on each axis (here 5x3 segments) and rename it to "Template"
- Select first Grid and run script Alt+P from Script editor.
- Script will generate Empty object for each face with Transformation Constraint - target to Template transforming location to rotation.
- script should set also rotation Max 180°, that is not happening for me so just after script when all generated empties are selected go to Constraints tab and set value 180. WIth right click choose Copy to Selected.
- Now you can add Displace Modifier to Template mesh and set any texture to control flipping. Cloud type for random animation, Blend type Linear or Diagonal for transition from side to side.
Step 2 - Video Sequencer
If you create some animation render this sequence in Black&White ...
- Add this animation (or image sequence) as a strip into Video Sequencer (you can even hide this strip)
- Add two images or colors strips you want to mix
- Select top one and under Strip Properties N > Modifiers tab > add Mask modifier and set your B&W transition strip.
If you are interest to go by this way I can extend answer how to create a BW mask for Video editor or use a Scene strip, but you are probably looking to easier solution. SO try to search for already done some BW transition or you would have to use other app for that.
Or closer specificities behaviour of transition so there can be other solution for you.