on Mac OSX use the keyboard combination CMD + SHIFT and drag with the mouse. On Windows it should be STRG + SHIFT and drag. That would resize the container of the image (a graphic frame) plus the image inside the container together proportionally.
If you know the exact width or height you want to resize the container (plus image), you could select the container and use the control panel for either width and height or even percentages for resizing.
If the small chain icon on the left side of the input fields is closed, you can do the resizing proportionally in one go, if you edit one of the two edit fields and the other value will follow.
You could also use the percentage fields to edit absolute values; just insert the number plus the unit name:
"20 pt" for 50 points or "50 mm" for 50 millimiters. InDesign will immediately translate this absolute value to percentages.
In Windows, I believe, it should be Ctrl + Shift and drag the corner.