What do you mean "it didn't work"?
You know, to receive assistance on computer stuff you have to tell us step by step exactly what you did and what happened and what didn't happen.
You go (on viki main site, not here in discussions) under your profile picture, on the top right corner. There is a drop-down menu. Don't choose "profile" but lower down, "edit profile".
Then you'll get a page like this one.
Click on the button I have highlighted in yellow, "Choose file". It means they want you to lead them to a picture on your pc which you want to put instead of the old one.
Before that, choose the picture, and make sure that the dimensions are as recommended, 220X220 pixels. (You can find it if you right click the picture and go to "Properties". And the size shouldn't be more than 400 kb. Also keep in mind that it will be cropped to a round. See the picture of the cat I chose, and how it was cropped to become round? Make sure that the picture has the subject in the middle and won't suffer with cropping.
So coming back to the viki page. When you click on "Choose file" and you locate the picture in your computer hard drive, you click "open" and then it will substitute it to the old one. Done!