Crediting a sprite sheet with no credit "license"

[Vipershark]

It's always a good idea to credit the ripper if you use their works, regardless of credit tag and regardless of whether they specifically want credit or not. It's a good rule of thumb to just credit everyone and not have to worry about it.

To answer your question though, in the specific instance where there is no credit tag and they don't ask for credit, giving credit probably isn't necessary. Is it good practice to do so and should you do it anyway? Yes.