1. An image processing apparatus, comprising: an image composition unit configured to generate a composite image by compositing first and second images; a specification unit configured to specify a generation process of the first and second images; a selection unit configured to select a compression parameter to compress the composite image based on the combination of the generation process of the first and second images specified by said specification unit; and an image compression unit configured to compress the composite image using the compression parameter selected by said selection unit, wherein said image composition unit generates the composite image by compositing the first image regarded as a semitransparent image and the second image.