First, you really shouldn't be using inline styles; you'd be better off creating a class in your stylesheet and applying it to your elements.
Second, we can't really see any other styling applied to these images so it's hard to say; however, based on what's available, I'd say the reason they're on separate lines is because you're adding 500px to the left margin of the second image. Because of this they both can't fit on the same line, causing to to fall to the second line.
Using a quick measuring tool (MeasureIt), you can see what I am talking about here: http://i.imgur.com/bl7ynV3.png. The "My Account" image occupies the entire bottom. Remember, margin is physical "space", meaning it occupies its space and will move other elements to make itself fit.