Although there are other news articles on this subject, it is still unclear to a lot of people how and what to suggest for a DD.
i will guide you through the process step-by-step
What kind of images should i suggest?
no absolute answer here, but you should suggest something that stands out from the crowd. Something with either an unique concept, quality or beauty.
Browsing the DD section for a while could give you an idea, though art and taste is subjective.
It is ok to suggest a friends piece, but don`t do it just because its a friend, do it if the piece is excellent.
Who am i gonna suggest it to and what should i write?
http://help.deviantart.com/18/ has the list of all the gallery directors and what category they are "attached" to, all you need to do is check the category the image is in, and take it from there.
Under the image you want to suggest is a thumb code ( under page views )
Lets say you want to suggest for a daily deviation, you copy the thumb co