I have number of images with alpha layer, with different size and aspect ratio. I need to bring them to one size(say 150x150) with maximum possible dimensions keeping alpha and aspect ration. For example - if image is 1500x1000 - then it should be downscaled to 150x100, vertically centered and empty 25px on the top and bottom should be filled with white color. If image is 1000x1500 - then it will be downscaled to 100x150, horizontally centered and empty pixels on the left and right filled with white.
Any tools for this( win or linux)? Thanks