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