THIN YOUR PAINTS
"THIN YOUR PAINTS" is far and away the most commonly offered painting-related criticism seen on /tg/, mostly seen in reference to some new Warhammer 40,000 player's fuzzy photographs of his first attempts to paint a model. The second most common appears to be "YOU PUT THE FUCKING PAULDRONS ON THE WRONG WAY ROUND", since new players are inevitably playing Space Marines. Trolling or not, "thin your paints!" is almost always good advice to give a new painter. However, the phrase has reached the point where it gets posted whenever something badly painted is shown on /tg/.
THINNING YOUR PAINTS
In all seriousness, the most basic paint thinner for your miniature needs is tap water, really that's it. Thinner paint basically means lesser consistency and the less its consistency is, the less messy it looks and the easier it is to manipulate while brushing.
The water:paint ratio pretty much depends on your preferences and needs; painting a single, normal-sized mini would probably just require you to dip your brush into a cup of water every time you apply a stroke of paint while painting entire squads or armies may warrant a bit of experimentation with the ratios and the creation of some sizable batches for convenience. Also note that more water = thinner paint, longer drying time, and too much water can ruin your paint's consistency entirely, so be careful when thinning large amounts or else you'll be wasting time and money.
The more intermediate thinners for painting minis are extenders and flow improvers, which are available in most art stores. They'll also cost you quite a bit if you're planning to paint an entire army, but on the upside, those products warrant much better results than ordinary tap water.
Gallery
A lot of the examples in the gallery were painted by this chucklefuck. Judging from the comments on his derptitude, it's pretty safe to say that on DeviantArt you can still get your e-penis blown no matter how awful you suck.
And somehow DeviantArt users found a way to get even worse.
This wonderfully crafted video depicts the correct way to react to these works of art.
<gallery>
Image:1237018138010.jpg|Decorated seventeen times and painted several more.
Image:1250644111790.jpg|THRONE'S ARSEPLATE, DO YOU HAVE PARKINSON'S OR SOMETHING?
Image:1292840989889.jpg|Oh dear sweet Jesus.
Image:1304388001751.jpg|Ultramarines truly worthy of their spiritual liege.
Image:1314090376649.jpg|This one's not a bad thumbnail - but take a closer look. Oh come on, stop screaming.
(Also, this is from a professional painting service. Yes, some asshole had the balls to charge money for this.)
Image:1314092204994.jpg|Don't look into their eyes jesus the captains gone cock eyed ahhhhhh.
Image:1314092361106.jpg|...THE GOGGLES DO NOTHING
Image:1302339865363.jpg|From My Little Pony's partial-birth abortion into your darkest nightmares.
Image:1323919239898.jpg|I HAVE NO MOUTH AND I MUST SCREAM WHAT THE FUCK
Image:1323919828242.jpg|I can't tell if Sanguinius is supposed to have a beard or if he's just vomiting.
Image:1314092488281.jpg|WHY ARE THERE BOOBS
Image:1323878987760.jpg|The guy on the right has two right hands. Mother of Christ.
Image:Sicarius.JPG|Sicarius, supreme Ultra Marine fighting for the spiritual liege. Do make note of his fabulous leopard print loincloth.
Image:1329292059994.jpg|BROTHER LIBRARIAN I CAN FEEL MY FACE SLOUGHING OFF (to be fair these are super tiny models. Those are pens that they're sitting on. Looks like a problem with the undercoat more than anything.)
Image:Yellowblob.jpg|By the GOD-EMPEROR. Did the guy stick his face inside a heavy flamer?
File:Meltingscout.JPG|Not THAT bad until you see the neck...
File:Forthegreatermelt.JPG|Why does it have golden locks?!
File:KNOWLEDGEISPOWER.jpg|Catachan is well known for its' unexpected and sudden Acid Rains.
File:ENGAGEALLTHETERROR.jpg|How could you fuck up the Battlesuit as well? That's like, an achievement all of it's own!
File:TRAITORSDOWNFALL.jpg|It takes a certain mindset, to join the vile Tau Xenos. But that Kroot gun with a chianblade baonet looks pretty cool.
File:Uglyshitwtf.jpg|Paint thinned? Maybe. Astrocious painting? Indeed.
File:6547473615 08152bc762 b.jpg|You can see the pain in his eyes.
File:Everfail.JPG|Don't look into his... eyes.
File:1369548189366.jpg|I am not an animal! I am a human being! I am a man!