Here are the image size guidelines: Image size is to be no larger than 1024 pixels on the horizontal or 800 pixels on the vertical side. File size is not to exceed 200 kb. Maybe when you are posting a portrait you are exceeding the height requirements?
I like the image you have posted. I think the blue spot is a lovely color but it seems to pull my eye. I might tone it and the perch down just a bit and run a round of sharpening on the perch if it were mine. Looking forward to the two birds post!!:)
Looks like you got the vertical posting figured out. On the first image the tree is a little too dominant, and cropping at least half of it away from the right would strenghten the comp.
The second is a nice opportunity you had. Is it cropped? If so I think adding back a bit top and bottom, then perhpas cropping a bit at left could provide an interesting vertical pano. As is it is quite tight top and bottom. Both bird's poses and HAs are perfect. I like the combination of tree trunk at right, and curved perch at bottom.
Another option, although a bit more extreme and perhaps beyond your personal ethics (but still releatively easy), would be to raise the whole bottom bird and it's perch then crop at bottom (and again add space above).