From your examples, I really like the 'DIY wall mural' mountains, 'winter trees', and '2 pc modern pines'.... But not all together, of course. :)

In general, I'd only hand-paint the vertical walls in this room. The exception would be if I decided to paint the vaulted portions like sky, or sky with just a bit of tree-tops -- but only with deciduous trees (in leaf or not).

Get rid of the wall paper FIRST. It's not that hard. Plenty of you tube videos exit. Peel off the "face" of the paper, spray/wet an area of a few square feet, let it soak in and scrape with a wide blade for that purpose..........gently. . Wash the wall well, and apply a PRIMER.

what jan said. its just not worth working around something like this. don't paint over it tho, that will just add work down the road when it bubbles and/or the seams always show.

i agree, tree tops, clouds, and birds on the ceiling, mountains would feel closed in.

looks like you'll need to open the door for ventilation; that means once your child is out of their crib you'll need a safer deck rail.... a non-climbable one.

I would embrace the wallpaper and paint the ceiling to match the wallpaper background (same color, lighter shade). If you want to add some color, paint the remaining walls blue. Otherwise, I would paint them dark grey. Can you post a close-up of the wallpaper?

Get rid of the wall paper it is not adding anything to the space. This is your new baby deserves a new space. I love hand painted murals on baby room walls and they can be quite simple and still amazing.These are a couple of easy to do ones

Here is what i did with my nursery

Love the architecture; very cool space!

A mountain scape would work really nicely with the geometry of your room! Murals are great whimsical features especially suited to a nursery. At the same time, it can feel a little forced and, in this case, may compete with the quaint wall paper. Looking at the pinterest board, consider some of the great accessories to achieve the mountain theme and incorporate the theme of the wall paper with additional accessories. Keep the wall color fairly neutral- in the beiges or greens-to add a timeless element to the space and allow the structural character to stand out.

Best of luck with your project!