imread() can import whole bunch of graphical data types, including .jpg and .png. Conversion to .png, as you ask, is not necessary.
img = imread('filename.png');
To alter the size/positioning of the image within your figure, you can touch the underlying axes object:
x = 0;
y = 0;
width = 0.5;
height = 0.5;
set(gca,'units','normalized','position',[x y width height])