Opening External images posible and not only PNG and JPG. If you want to use your c++ dll for GETing then you'll need decoding script to convert readed data from ext.binary, in to width, height, and data(pixels) according to mask of image format. Advantage of this is ability to use not only PNG and JPG images but allso BMP, ICO, PCX, GIF, and any other that you may need.
But you can use Fba's TCP or UDP sockets to download images from internet dirrectly to your app and inside of it, image code should be decoded. And internal sockets is not so hard as it seems from first look, I even used TCP in project to download protocols of other program via net, and UDP in my FbaGame for multiplayer (with PING and all other cool stuff...

).
Be Well.