I think that vine at the front door is beautiful. It just needs a good hard haircut to tame it down a little bit.
Putting posters/wall hangings inside your coop is a total invitation for a dusty mess. Better off to keep the walls broomable/cleanable (ie: flat). If you wanted to hang anything, I'd recommend a good mirror (make sure it's mounted REALLY well) to reflect what little light that tiny window will provide.
I'd wash it down as well as you could inside and paint it a light, glossy color. You'll be amazed at how much paint transforms a space. Prime it first to seal in all those past (unremovable) stains. Give the nestboxes a good hard scrub/rinse. Then seal them with linseed oil mixed 50/50 with mineral spirits. This leaves the natural look of the wood but protects it from moisture/poo and will make it easy to clean next time. I'd also leave those plank half-walls in natural wood, too - I think they're pretty. You can apply that oil with the poultry wire intact.
If you had an image that you liked, or even wording, you can simply print it out on your computer to the proper size (tape it together if the page isn't big enough), tape it to the wall with removable painters tape and use some carbon paper (I'm old enough to remember that stuff) from any office supply store to transfer the design onto your freshly painted wall using nothing more technical than a sharp pencil. Bam, instant color by numbers. Then you can use acrylic paints or even those oil-based paint pens (I get mine from Michaels) to fill in the lines. No artistic talent needed.
That space over the human table begs for a narrow shelving unit on the side to stack supplies and a nice "mural" about eggs over the table. I'd find some matching sealable storage bins (medium metal trash cans?) to put under the table on casters to put your feed and shavings into. If they won't fit under the table, put them under the window - no wheels necessary then. Seal the table with the linseed oil and maybe paint the table top for easy cleaning. Heck, I'd put bathroom ceramic tile on the top, but that's me...
BTW, I'd put some ventilation into that coop. I see you have a window, but depending on where you live that might not be the best option.
Hope this helps -
Here's my coop interior:
ETA: cool rainbarrel