Use PNG. In Photoshop, work with 16 bits per channel and save as PNG. In Wilbur, read this as a PNG Surface (File>>Open, select PNG Surface as the file type and your PS PNG under File name). If you get an error message along the lines of "attempt to read a PNG with 16 bits per channel" then you didn't select PNG Surface as the file type. Save the PNG data from Wilbur as PNG SUrface to get it back into Photoshop.
Unless you really want some other format, of course...