Resizing picture/images

HI,

Im trying to understand when pictures are re-sized, and also when/why browser 
chooses to create a new window when displaying a picture in a frame.

Im working with a layout of frames. When i initially show a picture in a frame using the <FRAME tag src=
the image is displayed so that it fits the frame, and with a hovering magnify glas which gives
opportunity to zoom in.

While instead having a file reference in the <FRAME tag src=document-name,
and having a <IMG tag in that document to display the same picture, the picture is displayed 
in its fixed shape (chopped to frame), and no option for magnify glas,

EXCEPT when displaying in the first frame, then it pops up in a new window uncropped since
it has now a full window. !?
 
what rules are there (if any), what have i missed ?

Br georg