A script that can be used with ScriptedWallpaper to pick a random image from Wallhaven and set it as background for kde.

installScriptedWallpaper.sh

rm -Rf tmp
sudo apt-get install  qt5-default qttools5-dev-tools kde-workspace-dev
mkdir tmp
wget http://kde-apps.org/CONTENT/content-files/156317-Scripted_Wallpaper.tar.gz -O tmp/scw.tar.gz
cd tmp
tar -xzf scw.tar.gz
cd Scripted_Wallpaper

mkdir -p build
cd build
cmake -DQT_QMAKE_EXECUTABLE=/usr/bin/qmake-qt5 ../src -DCMAKE_INSTALL_PREFIX=`kde4-config --prefix` 
make
sudo make install 

KwallH

A script that can be used with ScriptedWallpaper to pick a random image from Wallhaven and set it as background for kde.
All the code is released under public domain.

kwallh.py

#!/usr/bin/python

import os,re,pycurl,StringIO
from optparse import OptionParser
from random import randint

_WALLPAPER_PAGES_REGEX = re.compile("https?:\\/\\/alpha\\.wallhaven\\.cc\\/wallpaper\\/([0-9]+)",re.MULTILINE|re.IGNORECASE)
SEARCH_URL = "http://alpha.wallhaven.cc/search"
CATEGORY = {
    "general":4,
    "anime":2,
    "people":1
}

PURITY = {
    "sfw":4,
    "sketchy":2
}

def download(url,out):
    curl = pycurl.Curl()
    curl.setopt(pycurl.URL, url)
    curl.setopt(pycurl.WRITEDATA, out)
    curl.setopt(pycurl.USERAGENT, "Mozilla/5.0 (Windows; U; Windows NT 6.1; rv:2.2) Gecko/20110201")
    curl.perform()
    curl.close()
    
def get(url):
    buffer = StringIO.StringIO()
    download(url,buffer);
    return buffer.getvalue()

def getRandomWallpaperID(query,categories,purity,resolutions,ratios):
    c=0
    p=0
    for ct in categories:
        try:
            c|=CATEGORY[ct]
        except: pass
    for pt in purity:
        try:
            p|=PURITY[pt] 
        except: pass
    
    url = SEARCH_URL+"?q="+query+"&categories="+"{0:03b}".format(c,3)+"&purity="+"{0:03b}".format(p,3)
    if resolutions != None: url+="&resolutions="+",".join(resolutions)
    if ratios != None: url+="&ratios="+"".join(ratios)
    url+="&sorting=random&order=desc"
    page = get(url)
    wallpapers = _WALLPAPER_PAGES_REGEX.findall(page)
    if wallpapers==None: return None
    return wallpapers[randint(0,len(wallpapers)-1)]

def getWallpaperURLById(wid):
    return "http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-"+wid+".jpg"




parser = OptionParser( usage = "usage: %prog <options>\n" )
parser.add_option( "-i", "--instance-id", action="store", dest="ID", default="monitor1", help="" )
parser.add_option( "-c", "--categories", action="store", dest="CATEGORIES", default="general,anime,people", help="" )
parser.add_option( "-p", "--purity", action="store", dest="PURITY", default="sfw", help="" )
parser.add_option( "-r", "--resolutions", action="store", dest="RESOLUTIONS", default=None, help="" )
parser.add_option( "-x", "--ratios", action="store", dest="RATIOS", default=None, help="" )
parser.add_option( "-q", "--query", action="store", dest="QUERY", default="", help="" )

(o,args) = parser.parse_args()

instance_id=o.ID
working_dir = os.path.expanduser("~")+"/.kwallh/"

for f in os.listdir(working_dir) :
    fl=os.path.join(working_dir,f)
    if os.path.isfile(fl):
        if f.startswith(instance_id+"-"):
            os.remove(fl)

if not os.path.exists(working_dir): 
    os.makedirs(working_dir)

wallpaper_id=getRandomWallpaperID(
    o.QUERY,
    o.CATEGORIES.split(","),
    o.PURITY.split(","),
    o.RESOLUTIONS.split(",") if o.RESOLUTIONS!=None else None,
    o.RATIOS.split(",") if o.RATIOS!=None else None
)

wallpaper_url=getWallpaperURLById(wallpaper_id)

output_path=working_dir+instance_id+"-"+wallpaper_id+".jpg"
of=open(output_path,"wb")
download(wallpaper_url, of)
of.close();
print output_path