The monitor does not give us a resolution as this is being defined by your video controller. You could add 
tblVideoController to your report and let it display the current resolution:
Select Top 1000000 tblAssets.AssetID,
  tblAssets.AssetName,
  tblAssets.Domain,
  tsysOS.OSname,
  tsysOS.Image As icon,
  tblMonitor.MonitorManufacturer As Manufacturer,
  tblMonitor.MonitorModel,
  tblMonitor.SerialNumber,
  tblMonitor.ManufacturedDate,
  tblVideoController.CurrentHorizontalResolution,
  tblVideoController.CurrentVerticalResolution
From tblAssets
  Inner Join tblMonitor On tblMonitor.AssetID = tblAssets.AssetID
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
  Left Join tblVideoController On tblAssets.AssetID = tblVideoController.AssetID
Where tblAssetCustom.State = 1
Order By tblAssets.AssetName