diff --git a/src/tile.js b/src/tile.js
index a6d40fd8..582ccb67 100644
--- a/src/tile.js
+++ b/src/tile.js
@@ -54,8 +54,9 @@
  *      with HTML the entire tile is always used.
  * @param {String} postData HTTP POST data  (usually but not necessarily in k=v&k2=v2... form,
  *      see TileSrouce::getPostData) or null
+ * @param {String} cacheKey key to act as a tile cache, must be unique for tiles with unique image data
  */
-$.Tile = function(level, x, y, bounds, exists, url, context2D, loadWithAjax, ajaxHeaders, sourceBounds, postData) {
+$.Tile = function(level, x, y, bounds, exists, url, context2D, loadWithAjax, ajaxHeaders, sourceBounds, postData, cacheKey) {
     /**
      * The zoom level this tile belongs to.
      * @member {Number} level
@@ -131,11 +132,7 @@ $.Tile = function(level, x, y, bounds, exists, url, context2D, loadWithAjax, aja
      * @member {String} cacheKey
      * @memberof OpenSeadragon.Tile#
      */
-    if (this.ajaxHeaders) {
-        this.cacheKey = this.url + "+" + JSON.stringify(this.ajaxHeaders);
-    } else {
-        this.cacheKey = this.url;
-    }
+    this.cacheKey = cacheKey;
     /**
      * Is this tile loaded?
      * @member {Boolean} loaded
diff --git a/src/tiledimage.js b/src/tiledimage.js
index afb5d48c..f1737425 100644
--- a/src/tiledimage.js
+++ b/src/tiledimage.js
@@ -1530,7 +1530,8 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
                 this.loadTilesWithAjax,
                 ajaxHeaders,
                 sourceBounds,
-                post
+                post,
+                tileSource.getTileHashKey(level, xMod, yMod, url, ajaxHeaders, post)
             );
 
             if (this.getFlip()) {
diff --git a/src/tilesource.js b/src/tilesource.js
index 48cfe851..7a7e991c 100644
--- a/src/tilesource.js
+++ b/src/tilesource.js
@@ -639,7 +639,11 @@ $.TileSource.prototype = {
      *   let result = new FormData();
      *   result.append("data", myData);
      *   return result;
-
+     *
+     * IMPORTANT: in case you move all the logic on image fetching
+     * to post data, you must re-define 'getTileHashKey(...)' to
+     * stay unique for different tile images.
+     *
      * @param level
      * @param x
      * @param y
@@ -666,6 +670,29 @@ $.TileSource.prototype = {
         return {};
     },
 
+    /**
+     * Most tiles are cached because their 'context2D' property is null (otherwise no caching occurs).
+     * Then, their cache is uniquely determined by this key: this key should be different if images
+     * are different! Note: default behaviour does not take into account post data.
+     *
+     * A tile can have either context2D defined (TileSource.prototype.getContext2D)
+     * or it's context2D is set manually. In those cases cache is not used and this function
+     * is irrelevant.
+     * @param level tile level it was fetched with
+     * @param x x-coordinate in the pyramid level
+     * @param y y-coordinate in the pyramid level
+     * @param url the tile was fetched with
+     * @param ajaxHeaders the tile was fetched with
+     * @param post data the tile was fetched with
+     */
+    getTileHashKey: function(level, x, y, url, ajaxHeaders, post) {
+        if (ajaxHeaders) {
+            return url + "+" + JSON.stringify(ajaxHeaders);
+        } else {
+            return url;
+        }
+    },
+
     /**
      * @function
      * @param {Number} level