You have created a custom theme called my_theme within the default package and want to override the native
template file /page/lcolumn.phtml. You will need to place the file in the following structure:

A.
app/design/frontend/base/my_theme/template/page/lcolumn.phtml

B.
app/design/frontend/my_theme/default/template/page/lcolumn.phtml

C.
app/design/frontend/default/my_theme/template/page/lcolumn.phtml

D.
skin/design/frontend/my_theme/template/page/lcolumn.phtml

E.
skin/design/frontend/base/my_theme/template/page/lcolumn.phtml