Hi,
Looks to be leaking from the drain plug - see if the plug is loose, but don't be tempted to go mad trying to tighten it, It's a taper plug and you can crack the casing if you over tighten it. If that doesn't fix it you'll need to drop the gearbox oil out, put a little sealant on the threads of the plug and refit it and put some fresh oil in the box.........But before any of that, make sure you can remove the fill plug ( half way up the RHS of the box).
Stu