First step: Go to the 'Account' tab here in Yandere Community.
Second step: Go to "avatar"
Third step: Have your icon/image/profile pic link in hands, how you do that? Try this... 
NOTES: When you take an image from Tumblr for example, and open it in a new tab, the photo's extension will be .pnj which is not supported, so when you paste the link, change the photo's extension at the end of the link to .png, it will probably work, if it doesn't work, send me a message in the DM, I will take care of your case personally.
SECOND NOTE: (I know that this is already on "FAQ" but countless people are having difficulty understanding or finding it, so I separated this information into a single, easy-to-read post, so fewer people will face this problem. By the way... Guys, you need to read the "FAQ"; it's very important, okay?