Finalement je n'ai que trouvé que ca :
"Explaining the Appearance, Color Depth, of Published Application Icons"
http://support.citrix.com/article/CTX104504C'est un peu vieillot, ca date de 2004, mais ca s'applique jusqu'à la WI 3.0, et ca raconte que :
"
Any icon chosen for a defined published application of a MetaFrame Presentation Server environment is automatically converted to a resolution size of 32 pixels horizontal by 32 pixels vertical. In addition, the color depth of the icon is converted automatically to 16 colors (4-bit) with the choice of colors constrained to the default Windows palette."
Visiblement ca s'applique toujours avec la 4.0.