To resize an image:
Open image in MS Paint. It comes standard with Windows and everyone has access to it. Go to Menu: Image-->Sketch/Skew.
Change both horizontal and vertical percentages to the SAME value. You may have to adjust this and check the Menu: Image-->Attributes to see what the image size is. Adjust as necessary.
This is the quickest, easiest way to resize an image to fit within the guidelines.