It doesn't matter what file format the images are, as they will all get uncompressed when loaded into memory, which is why your 150KB file turns into about 4MB, as Gfk has pointed out.
If you want to limit the amount of memory that is being used, then you will need to load them off disk when needed and then purged from memory when no longer needed. Basically, only keep 2 images in memory at once - the current one (for display and use within your fade), and the next one. Once the next one is fully visible, null the last one (so it's freed from memory), and load the next one in ready for the transition. I would also use a GCCollect() call after freeing up the first picture to free the memory straight away.
If you want to limit the amount of memory that is being used, then you will need to load them off disk when needed and then purged from memory when no longer needed. Basically, only keep 2 images in memory at once - the current one (for display and use within your fade), and the next one. Once the next one is fully visible, null the last one (so it's freed from memory), and load the next one in ready for the transition. I would also use a GCCollect() call after freeing up the first picture to free the memory straight away.