How to dynamically generate images in a web application

I am trying to develop a web application which sells customized toy cars like hot wheels. I want to give user option to customize their own car. For example the user can select the color, wheel, body type and every time a user selects something it should dynamically generate image for that so that the user can see how the car looks like. I also want to give option to user for uploading cars they have and modifying their colors and see how the car would look like. How should i approach this and what technology/language is useful for this ?