I'm not totally sure what you want, but can't you just use a percentage on the image;

Code:
<img src="IMG.png" alt="" style="width:75%; height="50%;">
And for the second question, do you mean overlaying one image on top of another? If so I think you can do it in a couple of ways. You can use position, but I'd probably just go with a background image:

Code:
<img src="IMG.png" alt="" style="background:url('IMG2.png') top left no-repeat;">
Haven't tested it but I think it should work.