In order to use Gradient Wipe in a Premiere PRO 1.5 widescreen project the image file selected for the wipe must be interpreted correctly with the widescreen pixel aspect ratio (1.422 in your case). Two ways to achieve that:
1- Use an image file where the correct pixel aspect ratio is stored within the file: Photoshop files produced by Photoshop CS and later can do this when the image pixel aspect ratio is set correctly. I believe that older Photoshop files (7 and earlier) cannot do this.
2. The other way is to modify the Interpretation Rules.txt file found in the Plug-ins folder of Premiere PRO (mine is under the "en_US" subfolder, yours will be different) such that Premiere interprets 720x576 files as widescreen with the pixel aspect ratio of 1.422 by default. The file contains instructions on what to change to achieve that.