1. index.wxml

<image style="width: {{imagewidth}}px; height: {{imageheight}}px;" src="{{imagethirdsrc}}" bindload="imageLoad"></image>

2. index.js

//获取应用实例
var imageUtil = require('../../utils/util.js');
var app = getApp()
Page({
        data: {
            imagefirstsrc: 'http://bpic.588ku.com/back_pic/00/03/85/1656205138bbe2d.png',//图片链接
            imagesecondsrc: 'http://bpic.588ku.com/back_pic/04/07/63/28581203949ca9d.jpg!/fw/400/quality/90/unsharp/true/compress/true',//图片链接
            imagethirdsrc:'http://img1.gtimg.com/ent/pics/hv1/13/71/2061/134034643.jpg',
            imagewidth: 0,//缩放后的宽
            imageheight: 0,//缩放后的高
        },

        onLoad: function () {
            
        },

        imageLoad: function (e) {
            var imageSize = imageUtil.imageUtil(e)
            this.setData({
            imagewidth: imageSize.imageWidth,
            imageheight: imageSize.imageHeight
        })
    }
})

3.util.js

//util.js
function imageUtil(e) {
    var imageSize = {};
    var originalWidth = e.detail.width;//图片原始宽
    var originalHeight = e.detail.height;//图片原始高
    var originalScale = originalHeight/originalWidth;//图片高宽比
    console.log('originalWidth: ' + originalWidth)
    console.log('originalHeight: ' + originalHeight)
    //获取屏幕宽高
    wx.getSystemInfo({
        success: function (res) {
            var windowWidth = res.windowWidth;
            var windowHeight = res.windowHeight;
            var windowscale = windowHeight/windowWidth;//屏幕高宽比
            console.log('windowWidth: ' + windowWidth)
            console.log('windowHeight: ' + windowHeight)
            if(originalScale < windowscale){//图片高宽比小于屏幕高宽比
                //图片缩放后的宽为屏幕宽
                imageSize.imageWidth = windowWidth;
                imageSize.imageHeight = (windowWidth * originalHeight) / originalWidth;
            }else{//图片高宽比大于屏幕高宽比
                //图片缩放后的高为屏幕高
                imageSize.imageHeight = windowHeight;
                imageSize.imageWidth = (windowHeight * originalWidth) / originalHeight;
            }
        }
    })
    console.log('缩放后的宽: ' + imageSize.imageWidth)
    console.log('缩放后的高: ' + imageSize.imageHeight)
    return imageSize;
}
module.exports = {
    imageUtil: imageUtil
}