diff --git a/lib/appdmg.js b/lib/appdmg.js
index 2c4cc60..3ee0bd2 100644
--- a/lib/appdmg.js
+++ b/lib/appdmg.js
@@ -279,10 +279,14 @@ module.exports = exports = function (options) {
    **/
 
   pipeline.addStep('Reading background dimensions', function (next) {
-    if (!global.opts.background) return next.skip()
+    if (!global.opts.background || (global.opts.window && global.opts.window.size)) return next.skip()
 
     sizeOf(resolvePath(global.opts.background), function (err, value) {
-      if (err) return next(err)
+      if (err) {
+        const err2 = new Error(`Could not read dimensions of background image: ${err.name}: ${err.message}\nTo avoid this, give an explicit window size (window.size property) in your dmg specification file.`)
+        err2.stack = `${err2.stack}\nCaused by: ${err.stack}`
+        return next(err2)
+      }
 
       global.bkgsize = [value.width, value.height]
       next(null)