for example — "Marble Madness Promo Steven Sleeping Books.png"
Model Sheets: Episodename_ModelSheet_smalldescription.frm for example — "Open Book Model Sheet Connie.png"
Backgrounds: Episodename_Background_smalldescription.frm for example — "Rose's Room Background Rose's Room.png"
Drawings: Episodename_Drawing_smalldescription.frm for example — "Open Book Drawing Falcon.png"
Other: Episodename_Name_'smalldescription'.frm for example — "Open Book Celebration Meal.png" or "Open Book Color Key.png"
Profile/Blog/Thread/Template/Article Images: No special rules, just try to descibe the image as you can. for example — "Steven Sugar.png" or "BathMetal Profile Pic.png" or "Emerald Fanart.jpg"
PS: These rules are not strict. No one will be banned (except for people who make inappropriate names for the images), but will get a message on his/her wall if he/she will upload a lot of non-standardized images.
I've pretty much have always been doing this, but without spaces. Spaces make it difficult to find the photo later, since you have to remember that _ is in the url instead of spaces, and if you manually type photo names into wikitext you need the underscores or else the image does not show.