Implemantation of TPicture->Assign

I need to know exactly what take about the object passed to Assign
the TImage o TBitmap

What appens in the function

Assign(TPersistent Source)

when it is used with bitmap image?

For example I need it to teach OpenPictureDialog to open a my image-file.
I suppose it use something picture->Assign(MyModule);

Thanks, Mauro