That's strange! Do you see this happening across browsers/devices? Keep us posted.
@TeamYouTube Every time I try to update my Channel artwork this happens 
I’ve tried all week. Can you help me with a fix?pic.twitter.com/RGhVXqASZl
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Thanks. Twitter will use this to make your timeline better. UndoUndo
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Coach, blogger & co-founder
Just Start Now podcast