I'm trying to keep it simple. This certainly isn't the most flexible approach. In particular this might not work well if, for example, a green shoe doesn't have an image for black laces. You would need to have an image for every possible combination.
Make two functions, getShoeColor() and getLaceColor(). The functions should look at the proper select menu, and then return a string for whatever is chosen in that menu.
Then, you can use a naming convention for your image files.
So, for black shoe with white laces
In your showimage() function, you can call the other two functions to get the data you need to craft the proper filename. Just concatenate the strings together.
var shoeColor = getShoeColor();
var laceColor = getLaceColor();
var filename = shoeColor + "_" + laceColor + ".jpg";
//uncomment to test the value if needed
Give it a shot. Make sure you use unique names, and don't forget the use the onChange event for the other select menu.