You can use a black‑and‑white bitmap image as a transition mask. The first clip replaces the black area in the image, and the second clip replaces the white areas in the image. If you use a grayscale image for the mask, pixels containing 50% or more gray convert to black, and pixels containing less than 50% gray convert to white.
Adobe Premiere Elements can use any importable grayscale image as a gradient wipe. In a gradient wipe, the second clip fills the black area of the grayscale image and then shows through each level of gray as the transition progresses until the white area becomes transparent. When you create a Gradient Wipe transition, you can specify the softness of the transition’s edges.