Using so many and so large pictures on mobile, with Flex... it's not surprising you have performance problems.
First thing, you should adapt the images size to the device, on server side, if it's possible. Loading 36 HD images with a 3G connection will take a rather long time. And if it's to reduce their size on the device, it's a loss of time.
Then ensure you use a spark.components.Image and not the old Image class.
Last but not least, use a cache. It's not clear in the code in your question if you defined one, because the way the variable ldr is initialised is not shown. It should be something like:
var ldr = new ContentCache();
ldr.enableCaching = true;
ldr.enableQueueing = true;
Alternatively, if the images aren't supposed to change, you can embedd them in the application, instead of loading them, and use BitmapImage components to display them. The app will be heavier, but you won't have any loading time.