I had a close look at this problem and tested it with a Samsung Galaxy Ace and a couple of other devices. It turns out, that you should not use jpg files at all, because they have a longer loading time and take up much more graphics memory than pngs. Also, please keep in mind to create 3 different image versions for sd, hd and hd2 devices and using the MultiResolutionImage component. The Ace is in the sd (=single definition) category, which works best with images made for 480×320 or 320×480, depending on your game being in landscape or portrait mode. We also added documentation about this topic here: http://felgo.com/doc/vplay-different-screen-sizes.html
The best performance can be achieved by packing all images into a spritesheet, with the excellent tool Texture Packer. By using the spritesheet, your image only needs to be loaded once into memory and can then be used efficiently.
In addition, we also added a new function to the NativeUtils component called clearCocosTextureCache(). This allows you to manually remove cached textures that are currently not in use. You can use it, if you know you will not load the images anytime soon, and it will be available with the next Felgo update v1.3.
Please let me know if I can help any further,