The picture on the left is a close-up of the wall of a house taken on a cool, cloudy spring morning. The picture is unremarkable and doesn't tell us much.

The second image is of the same section of wall but taken with a Thermal Imaging camera.
In this picture, different colours correspond to different temperatures as indicated by the colour scale on the right hand side of the picture.
Yellow regions are the warmest in this picture. Orange areas are a little cooler and purple areas are cooler again.

We are most interested in the wall because we are concerned with heat loss from the house through the wall.
Most of the wall is purple which means it is relatively cool. The section under the window is orange so that section is warmer than the rest of the wall.

Why is that so? Why is that section of the wall warmer than the rest?
As it happens, there is a radiator situated inside the house below that window.
The orange area in the thermal image tells us that heat from the radiator is coming straight out through the wall!
A large proportion of the heat emitted by the radiator - perhaps as much as 50% - is immediately lost through the wall.

How would you solve this problem?
Insulation. In this case dry-lining internally with insulated plasterboard would help keep the heat from the radiator in the house.
Much less heat would then be needed to keep the house at a comfortable temperature so less oil is needed and less money is spent on fuel.