Ok, i only have so much experience working with jpg/png etc..so if theres someway easier or better then i am doing, please feel free to pitch in ok.
I haven't had a rotating screen in a while, I been using a low resolution easy as pie to pick an use different wallpapers each day an non of that, looks odd feeling.
So normally, i find a picture i like an just resize it in paint or Slowview.
Now,its moving on the IQ/Expo, the High Resolution, combined with a rotationg display screws things up a bit for me, maybe not you, but me ok, lol.
Now, its Res is 480x800 in portrait, ok no problem, but the rotated display is now 800x480, so say you were looking at a picture of wine bottle in portrait in 480x800, now in landscape your looking at the label only really.
Which for some pictures is fine, but for most its not, an some trickery is lost.
So, for myself an my wallpapers, i use pictures now that are 800x1280/ 800x1200, i know it sounds odd, but hear me out an show me a better solution if ya gots it ok.

To use that picture of 800x1280 or even 800x1200, i use Wm Picture&Videos media viewer, select the picture,goto menu, use as desktop background.
Now, it gives you a option, of making it wee bit transparent and(Pass,why lose the clarity,unless your after that.), where you want to look at your picture in portrait mode, ok, take the selection box an slide it to the far left, then select ok. Now when in portrait, i see what i have chosen in portrait, an when its slid open, you see the whole picture.With the right picture, you could have it completly black in portarit, an showingmore in landscape.
I'll attach a sample resized picture to try if you like.
Ok, now,, what are you all doing for yours with your wallpapers?
Peace
WR