Sorry, we don’t expose any asset management capabilities like that. You could remember how many images you have loaded so that you can count them down in a succesFunc, if you first add extra work to the Picture.source data binding to remember how many:
var picturesToLoad = 0;
...
new go.Binding("source", "src", function(s) {
picturesToLoad++;
return "images/" + s + ".png";
})),
...
// later picturesToLoad--; in the success func, then check for 0 again.
I think that will work.
Otherwise, you can use the callback to makeImageData as Walter says.