Saving/loading document state quickly and robustly for image editor

Question

I'm looking for some critique on my approach for storing the state of a bitmap editor for Android and iPhone mobile phones. Even a "Looks fine to me!" response would be great!

In the application, the current user document contains several bitmap layers (each maybe 1024 by 768 pixels) that can each be painted on. The basic requirements for the application are:

1. I need to be able to save and restore the document state.

2. When the user quits the application or gets a phone call, I need to be able to save the document state quickly (within about 2 seconds).

3. If the application crashes, I need to be able to restore the document state (it's OK if the user loses maybe 30 seconds of work though).

For 1, I cannot find any open file formats that support layers. I was going to go with the following file structure for storing my document:

document_folder/
  layer1.png
  layer2.png
  ...
  metadata.xml

The layers are just stored as .png files and the .xml file contains data such as which layers are currently visible. The document folder can either be opened as is by the application or the folder can be stored in a .zip file. This seems like a nice simple format for other applications to work with too.

In addition to .png files, I will also allow layers to be saved in a custom .raw file format which contain unprocessed raw pixel data from bitmaps. I can save these very quickly on the phone (<0.5s) whereas .png files take a second or two.

My plan for quick-saving the document was, on start-up, to create a folder called /autosave, and save .raw versions of all the layers there. After a few editing commands on one layer, I would then update the .raw file for that layer in a background thread. For robustness when saving, I would save the layer as e.g. layer1_tmp.raw and when I've confirmed the file has been fully written, replace layer1.raw with this file.

Should the application crash during use, I would just reopen the /autosave folder. When the application is closed or the user gets a phone call, I just have to update the last modified layer to autosave. When the user wants to save, I just convert all the .raw files to .png files and then zip the folder.

What do you think? Are there any obvious flaws? Is there a simpler way? Am I reinventing the wheel somehow? Thanks.