As was said in another question, using the drawable-nodpi folder will prevent android from resizing your pictures.
Additionally, if you want to have multiple versions of an image for hdpi and ldpi format, but don't want the system to resize them (to keep a power of two resolution for example), you can use the following code while loading your bitmpap :
// ask the bitmap factory not to scale the loaded bitmaps
BitmapFactory.Options opts = new BitmapFactory.Options();
opts.inScaled = false;
// load the bitmap
Bitmap bitmap = BitmapFactory.decodeResource(ctx.getResources(), R.id.mybmp, opts);