There is a roundabout way of doing this. YOu can use the following steps to do this.
1) Go to Edit->Preferences
2) Select Email from the list on the left side
3) In the list that displays email clients, select "Other(Save to Disk)" options
4) Click OK
5) Select the image that you want to reduce the resolution of
6) Click File->Attach to Email
7) In the email dialog, click Individual Options
8) In the size, you can select the resolution or provide a custom resolution.
9) Click OK and a dialog box will be displayed which will give you the path where the file has been saved.
I know this is not too direct but is helpful when I want to make my pictures smaller.