Go to admin > CMS / Page and Text Manager > [the page you want to edit].
Right-click on the image you have linked and select "Image Properties".
Click on the "Visual settings" tab and enter 0 in the "Frame" field.
Then click "Update".
Instructions for you who have version 9 and later.
-----
Go to admin > CMS / Page and Text Manager > [the page you want to edit].
Right-click on the image you have linked and select "Insert / Edit Image".
Click on the "Appearance" tab and enter 0 in the "Frame" field.
Then click "Update".