Tom
Icon should be 32 x 32 pixel in order to be pasted in a script property dialog box. The way I usually do is first to make an icon that I like on VW and export it to image (bmp file) by drawing a marquee around it. You have to assign your image size by 32x32 pixel when you export. Then you open this in 'Preview', and copy. Open your script editor and select the tool, select property, click on the icon box, and paste.
I never had any problem of this procedure.