Let me see if I understand this.
You have an image (lets say a picture you took) on your phone. When you're holding your phone upright (portrait) the image is quite small and there are back bars above and below it. You want to have it fill up your whole screen so you flip the phone sideways (landscape). You want to keep the image full screen, but view it in portrait.
if I'm understanding you correctly, what you're asking isn't possible. If you want to view it full screen, it will have to be viewed in landscape, because the image will not fit the screen if viewed in portrait. That's why there are the back bars - to view the whole image in portrait, it has to be shrunk down so that the wide image fits the narrow side of the screen. The back bars fill up the left over space. The other option is to zoom and crop a narrow strip of the image so it fits the screen when in portrait - but this will cut off the ends of the image.
To get what I'm saying take 2 pieces of printer paper - they're the same size right? but if you lay one sideways (landscape) and one upright (portrait) you can see that there is overlap one way or the other. The only way to make them fit together is to cut the ends off one (leaving the narrow strip that fits in portrait - but you lose some of the image) or to shrink one down so it fits inside the portrait paper (you get the whole image - but it's much smaller and you'll have the back bars above and below.
Unless I'm mis-understanding you, I don't think it's possible. Unless you use a photo editor to rotate the image and re-save it in the new orientation - but I don't think this will work either, again unless I'm totally mis-understanding what you're trying to do.