Most Carrelet page layouts include a logo. When you first activate the account, this is set to a rather dull default image. You can replace it with another image at any time. Carrelet will accept images as jpeg, gif or png files. Most scanner software can produce files in at least one of these formats.
You should check that the image file is not too big (30k is usually considered an upper limit for web images, and Carrelet will reject really huge files). You should also check the size (in pixels) of the image: for most applications, 200 pixels wide is a sensible maximum.
If you are using gif or png formats (which is recommended for line art-type images), consider setting the background colour of your logo to transparent, so that it will look right on whatever colour background you choose.
Then click on the browse button on the Carrelet control panel in the row containing the text Upload image file called ... for all pages ('logo') and select your file. Click Go and the file should be uploaded to the server.