1.获取图片大小

  1. function getImageWidthAndHeight(url){
  2.          let img=document.createElement("img");
  3.       img.src=url;
  4.       return [img.width,img.height];
  5. }

2.图片等比例缩放

  1. function AutoSize(imgOld,newWidth,newHeight){
  2.          let imgNew=new Image();
  3.       imgNew.src=imgOld.src;
  4.       imgNew.width=imgOld.width;
  5.       imgNew.height=imgOld.height;
  6.       let scale=imgNew.width/imgNew.height;
  7.   //确保参数的正确性    
  8.   if(imgNew.width>0 && imgNew.height>0){
  9.     //当图片宽高比比要求的宽高比大，newWidth拉满 
  10.     if(scale>=newWidth/newHeight){
  11.         ImgOld.width=newWidth;
  12.          ImgOld.height=scale*newWidth;
  13.          }
  14.         //当图片宽高比比要求的宽高比小，newHeigth拉满 
  15.     else{
  16.         ImgOld.height=newHeight;
  17.          ImgOld.width=scale*newHeight;
  18.         }
  19.    }
  20. }
  22. let img=document.querySelector("img");
  23. img.onload=AutoSize(this,100,100)

参考文献：
图片等比例缩放图案
链接