can anyone tell me if these belong to a manga or just a random drawing from a specific artist?

http://www.mangago.com/read-manga/uwasa_no_oujisama/mp/manga/Uwasa-no-Oujisama/c1-06/39/
http://www.mangago.com/read-manga/goshujinsama_to_wanko/an/goshujinsama-to-wanko-chapter-6.html/2/

Hm, I think the first one is from the same manga you linked, Uwasa no Oujisama. The character is Taku-chan, Yuu's best friend and in this page you can see the mangaka says "will anyone think Taku-chan is really handsome?", so that's probably just a extra illustration, like a special page where the mangaka does a sketch and some ramblings... Colored by Attractive Fascinante team.

For the second one, is probably a fanart or a random illustration, since it doesn't look like a manga or anything. I could even try guessing it's a fanart based on Kateikyoushi Hitman Reborn! characters, but since I never read nor watched it, I can't say much.

Thanks so much! XD

The second one is a One Piece fan art. They are Eustass Kidd e Trafalgar Law 😃