So I think what you want is to import an image to use as the background for your application in Catalyst, and make that image repeat?
You can do this by:
1) Click the 'File' menu
2) Select 'Import', and then 'Image...'
3) Select the image you'd like to use as your background image
4) You will now see your image in Catalyst, and it will appear under the 'Images' section of the Library panel.
5) Click the image to select it
6) Look at the Properties Panel in the bottom right hand side
7) You will see an option called 'Scaling:' which is set to 'None' by default
8) Change the 'Scaling:' to be 'Tile' instead of 'None'
You can now change the width/height of your image by scaling, and see that it now repeats.
Maxime, does this answer your question? Let me know how if it works out for you.
Je peux parler un peu de francais, alors si vous avez besoin d'aide tu peux m'envoyer une e-mail.