Thanks, lots!You'll need to get a flash decompiler and go through all the images in it until you reach the one that you want to change.
(Didn't do this myself, but it should be something like this)
Step 1: Download the flash decompiler
Step 2: Open it, and drag your Habbo.swf into the decompiler
Step 3: Open the images folder on the left and search for the image
Step 4: After finding the image you want to change, change it
Step 5: Make sure you save it before exiting
Step 6: Replace your .swf file, and clean your cache (only if its the same name).
Link to a free Flash Decompiler:You must be registered for see links