Go here: http://www.pixelmator.com/try/
to download the trial version of Pixelmator (unless you already have a the full version)
1. Open Pixelmator and hit "Create new image…"
2. Enter the size you want and hit "OK"
3. You will get a new image with white background
4. Hit Command (cmd) + a to select the background then hit the delete key to get rid of the white. Now the background is transparent.
5. Drag&Drop your vertical image into the new image. You can then move it around to center it. View=>Show Grid may help. I wonder if there is an easier way to center the image. Never used Pixelmator before sorry. If you find out let me know.
6. Click on the cogged gear on the top left of the new image window and choose transform or go to Edit=>Transform to be able to resize the vertical image you added
(when resizing choose one of the angles and click and hold shift while resizing)
then hit OK
7. Go to File, Export, choose PNG and you will have your vertical image with the same size of a horizontal image with transparent background.