corona SDK image

image_proportion.lua

--Function to get the New Height for Given Width
function getNewHeight(image , newWidth) 
    local r = image.width / image.height --4
    local newHeight = newWidth / r --5
    return newHeight --6
end

--Function to get the New Width for Given Height
function getNewWidth(image , newHeight)
    local r = image.width / image.height
    local newWidth = newHeight * r
    return newWidth
end

--Usage
function createAnImage()
    local myImage = display.newImage("myImagesDirectory/MyImages.png") --1
    -- Call this before you are setting the width of the image
    myImage.height = getNewHeight(myImage, display.contentWidth * 0.90)  --2
    --Set the same width you used in the previous line
    myImage.width = display.contentWidth * 0.90  --3
    sceneGroup:insert(myImage)
end