You can see me in the lower left-hand corner of the picture.
いつも隅に引っ込んでいてはだめよ
Don't just hide [stay] in the corner all the time.
隅から隅まで捜したのか
Have you 「looked everywhere [searched every nook and cranny]?
そのことを隅から隅まで知っている
I know it like 「a book [the back of my hand]./I know every detail of it.
あんな美人の彼女がいるなんてあの男も隅に置けないな
He must know a thing or two to have such a good-looking girlfriend./There must be more to him than 「meets the eye [we think] if he has such an attractive girlfriend.