Element's alternate image - dimensions

[Martinigue]

Hello all,
I'd like to have a certain degree of control over the size of the images produced by the documentation generator; what I do : design a UI flow, then I make each screen a composite, to its embedded diagram I add a class element to which I set an alternate image - the snapshot of the actual screen / window. However, when the documentation gets generated, the images get scaled, oftentimes they get differing dimensions and include unnecessary text ( name of the class element, etc..).. My question is - is there somewhere that I couldenforce the size of the geneterated images? How can I make sure that if I set an alternative image to an element, it will be of anticipated size in the diagram ( the current scale factor of the diagram has influence here as well) ?