The ornamental design for a transitionalimage for a portion of a displayscreen, as shown and described.
FIG. 1 is a front view of the first image of a transitional image for a portion of a display screen showing my new design; and,
FIG. 2 is a front view of the second image thereof.
The appearance of the image transitions sequentially between the images shown in FIGS. 1-2. The process or period in which the first image transitions to the other forms no part of the claimed design. The broken line showing of the portion of a display screen in both views is for illustrative purposes only and forms no part of the claimed design.