When you add a custom transition, you need to add a black and white image to work as a transition.
Add a custom transition
Select a video clip you want to add a transition, click Transition and add a transition named Customize.
Select a transition direction and select a black and white picture on the computer, and finally click "open". A custom transition is added.
Reset a custom transition image
Select a custom transition and right click "edit". This will open the properties that you edit of the transition. You can edit the duration, softness, reverse, etc. of the custom transition like you do with a built-in transition. In the Type option, you can change the transition image.
When it is a custom transition, the name of the image that is currently used for the transition will show in the Type.
Click "Add Image", you can select another black and white picture from your computer to replace the current image. After changing to the new image, you can see that the name of the new image showing in the Type, indicating that the custom transition image has been successfully changed.
Also, click the image name to expand a drop-down list, from which you can select the images that were used for transitions.
Remove a custom transition image
Click "remove image", the currently selected transition image is removed from the current transition as well as the drop-down list of transition images.