Apparently it's not common knowledge that Pixiv has a ton of guro. Let's make it common knowledge, then. I'll show you how to find it.
Create a Pixiv account if you don't already have one; log in. You should end up on a page like this.[1]
Hover your mouse over "settings" in the top right. Select "user settings." Like so.[2]
In the "user settings" page, beside "browsing contents" halfway down the page, you'll see your filters. They ask if you're over 18. If you tick "yes," you get the other two options: R-18 and R-18G. Choose to show both. The page.[3]
Click "apply," and you're good to go. You can[4] now access[5] content you[6] couldn't even[7] see before.[8]
Not sure how to find artists who draw guro? I recommend checking the bookmarks of artists you already follow who draw guro. Not following anyone? Let's remedy that.
Guro artists on Pixiv: Andlex (me; I draw bloody, relatively quick deaths)[9]
faithstudent (does photomanipulation and bloody death)[10]
yamaton1gou (does Gmod guro; it's hotter than you'd think)[11]
frederick (hand-drawn hard BDSM and some guro)[12]
NORA (high-quality 3D cannibalism-focused guro and BDSM)[14]
royalhouse (hard guro, mostly on the same woman)[15]
gotyui (mostly naked lolis, some guro as well[17]
moodame (highly stylized guro and scat)[18]
As109 (high-quality sex sequences, some guro)[19]
zofe13 (cute, colorful girls; bloody, brutal deaths)[20]
gutoku (bloody deaths, cute facial expressions)[21]
g2 (high-quality guro; last update was a long time ago)[23]
vestman (guro, with a focus on a particular character and plenty of decomposition)[24]
mikawamakiba (high-quality guro; considered by many to be the current king of guro)[25]
pphinani (cute girls, gory deaths)[26]
vvindowsme (pixel art guro animations featuring adorable lolis)[27]
sukiyo (stylized, high-quality guro)[28]
Ayaswan (mostly game girls; lots and lots of blood)[29]
Probably 80% of the pictures I've posted till now (October 22, 2013) came from Pixiv. It's a goldmine, guys and girls of r/guro. Use it well.