NB: The process I describe here may not be the most efficient way of creating the WAD
but it works, so I'll stick with it.
Start Wally and select File -> New from the top menu bar.
In the Create New Texture dialog box that appears, change the Type:
to be Half-Life Package [wad3] [.wad] then click OK. A new untitled WAD package
Go back to the top menu bar and select File -> Open. Find your
BMP file and Open it. Your bitmap image will appear in its own window.
Right-click on this image and select Copy then select the Untitled WAD
window and go to the top menu bar.
Select Edit -> Paste -> Paste Into Package
File. In the dialogue box that appears enter the name as LOGO and click OK.
Your texture will now be visible in the WAD package.
With the WAD window still selected go back to the top menu bar and select File
-> Save As and save the WAD somewhere as pldecal.wad.