The two sql:

Number 1:
SELECT * FROM conteudo_imagem WHERE idConteudo
IN (SELECT idConteudo FROM conteudo WHERE idGrupo = 9) ORDER BY idImagem

Number 2:
select conteudo_imagem.*
from conteudo_imagem ci
inner join conteudo c
on ci.idconteudo = c.idconteudo
where
c.idgrupo = 9
order by ci.idimagem

The questions:

The two sql resulting at the same result?
What of them is more performatic?

i believe you will find that number 2 will give an error

instead of select conteudo_imagem.* you need to say select ci.*

otherwise, yes, both will give the same result

which of them performs better? that depends on your database management system

this is the ANSI SQL forum, which is more concerned with correctness of the SQL than performance

Thanks !!!

There is actually a third equivalent formulation, sometimes more performant than the two others:

Not only on the RDBMS, but also on the current status of the two tables (i.e., number of rows, width of the rows) and surroundings (presence of indexes, physical ordering of the data, ...)
Even with the same tables on the same RDBMS, but at different time instances, a different one from the three formulations will be the most performant one.