this should work:
Select Top 1000000 tblAssets.Username,
tblAssets.Userdomain,
Coalesce(tblADusers.Displayname, tblAssets.Username) As Displayname,
tblAssets.Lastseen,
tblAssets.AssetID,
tblAssets.AssetName,
Coalesce(tsysOS.Image, 'notscanned.png') As compimage
From tblAssets
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Left Outer Join tblADusers On tblAssets.Userdomain = tblADusers.Userdomain And
tblAssets.Username = tblADusers.Username
Order By tblAssets.Lastseen Desc