It is here:

http://seiga.nicovideo.jp/

I can't find it in any dictionary. My guess is "quiet picture"

I thought silent movie when I first saw it, then I saw it was Nico and I assumed silent video. It looks like manga and magazine videos.

still picture

well 静止画 is still picture so maybe they misremembered it? i don't know.

wait it's right in the description
ニコニコ静画は「イラスト」を投稿したり、「マンガ」「電子書籍」にコメントを付けて楽しむことのできる、『静止画』をメインコンテンツとした総合サイトです。

I'd reckon it's the opposite of 動画. "Silent movie" is サイレント・ムービー or 無声映画.