not suupport if statements?

[juxiaoming]

Configuration does not support writing in if statements？

./nginx -t :
nginx: [emerg] "image_filter_watermark_width_from" directive is not allowed here in /usr/local/nginx/conf/nginx.conf:44
nginx: configuration file /usr/local/nginx/conf/nginx.conf test failed

[juxiaoming]

location /images/ {
root /usr/local/nginx/html;
index index.html index.htm;
set $path "/usr/local/nginx/html/water.png";

        if ( $arg_download = "download" ) 
        {
           # set $nameaa "/usr/local/nginx/html/empty.png";
        }

        
        image_filter watermark; #开启水印 
        image_filter_watermark $path; #水印文件位置 (绝对路径)
        image_filter_watermark_position top-left; #水印位置 
        image_filter_jpeg_quality   95; #图片质量 
        image_filter_buffer         100M; #缓存 
        image_filter_watermark_width_from  40 ;   # 打水印的图片最小宽度，只有大于这个宽度的才会打水印 
        image_filter_watermark_height_from  40 ;  #打水印的图片最小高度，只有大于这个高度的才会打水印
        
    }

and if $arg_download = "download" ,it will be 415 Unsupported Media Type