I'm back with news (good I think).
All the images stuff is working!
Thanks to Object Arts, and all the people who did the GDI+ implementation for Dolphin (SW, CU, LS, DA), it's extremely full featured and very Smalltalk.
As usual with a good Smalltalk, I searched through the class library and found exactly what I needed. It happens that GdiplusImage implements a method named #asByteArray: , which expects the MIME type string (i.e. 'image/png'), using that for the encoding, plus four or five methods, it began to work. Without considering the side effect of polymorphism/dynamic binding, because all the Seaside (and Squeak) expects Forms as it base for all the image/pictures working, I use Gdiplus images and both understand a minimum common protocol, including #width, #height, etc.
However, it's working!.
I've found some other (worst?) problems, but all will be fixed, in their time.
I'll keep walking on the seaside, until the sun falls...