If your sketchbook icon has a strange line of data code below it, showing the pixel size and the file size itself, then you need to reupload your sketchbook icon in 100x100 pixel.
This fixes the problem right now.
Also make sure that your icon is the only attached image. Sometimes it works with more images uploaded in the first post as well, but most of the times it causes problems.
The issue is reported and we're working on it.