This is a guide on how to do it:
FM Graphics - How to make XML config files
To summarise it, you need to have two images for your club logo. One is 180x180 pixels, and the other is 25x25 pixels. Both images must be .png files! So put both these images in to your graphics folder (create one if you don't already have it). Then, download that empty config file in the guide and place it where you placed the images. Then, edit the config file and type this between the "Auto generated by fmXML" and "list" lines:
Replace "Picture" with the name of the image (180x180=logo, 25x25=icon).
Replace "ID" with the club's unique ID, Constancia's being 814668.
After you do that, save the file. Then go to your FM preferences and "Clear Cache" and "Reload Skin".