First, have an image to insert in image format (not word, pdf, etc... but .emf, .jpg, .bmp, etc...)
Example :
Open the FicheDePaie.ffs or other format. Once opened, click on File - Save As and give it a new name (e.g. PaySlipLogo). This will keep the original format in case of problems.
Click on the button 
and then in the area where you want to place the image:
Select the image. Click on Open.
Select the appropriate options. Click on OK.
Select the appropriate options. Click on OK.
The image is placed on the format. It is possible to move it with the mouse, as well as to resize it by clicking on the red dots.
