umm.. to be safe, i wouldnt do it. not without asking.
i know there are some pieces of my artwork i dont want people toting on their blogs in a self descriptive sort of way. a lot of my art is personal to me and involves my own original characters, so it feels weird to see people using them and i dont really like it. although, stuff like reblogs and likes and sharing my artwork with others really flatters me, thats what i want! or else i wouldnt post it online. but when it comes to making a layout or an about page or anything related to creating something out of someone elses work, it sort of becomes your own thing that describes you, am i right? so thats what makes me uncomfortable. i dont really want my art used like that. a lot of people may feel this way or something similar about their artwork being displayed by other people this way, and i think its totally valid for an artist to not want that. but also, a lot of artists would probably encourage you to use their artwork however you like if you love it, so:
asking permission i think should be mandatory, for things like icons and making abouts/layouts/etc, especially when an artist is on tumblr or a similar art sharing and producing website where they are very easily accessible to you to contact. anything that has to do with YOURSELF rather than the artist. also when it comes to something like.. suppose youre shy or theres a language barrier, i think respecting the creator is much more important than whatever excuse one might have for not asking permission.