This may be your problem. Difficult to tell from your writeup. If the image you are building has the same name as an image already in the browser cache, it will not download. If this is the case, here's a workaround I picked up from another forum and it worked for me.

Create an image tag for your html that looks something like this.

  <img src="imagename.ext?randomcode"......
[download]

Where the random code changes with each new image. This will force the browser to update the cache.