i was really impressed by the thorough gallery search here so i have adopted my tagging system here from my tumblr for ease of searching for certain artworks.
this tag system also applies to art posted on my persona account casualdea

subject type

#fanart - art made based on another media/characters. this includes other people’s ocs (for artfight attacks as seen on casualdea)
#mayorsocs - art made based on my own characters. this phrase is appended to the character name on top of a tag with just their name (such as: #mayorsocsalice), because my tumblr habits of putting multiple words in a tag precede me.

medium

#digitalart - art made with digital tools
#traditionalart - art made with physical tools

tagging characters/fandoms

~ fandoms will be tagged with multiple common variations and abbreviations of their title (#fe, #fireemblem, #fireemblemawakening, #feawakening). this system may be inconsistent across all art, though
~ characters will most often be tagged with their full name, if applicable (#hatsunemiku). sometimes their first name solo (#miku) but i don't always remember.
~ if a character has no last name and has a rather generic name, then their name will be attached before their home media on top of their solo name (#olivervocaloid, #robinfireemblem).
~ my ocs will be tagged with solo names (#xane), full name (#xanebartlett), and with the mayorsocs attachment as stated earlier (#mayorsocsxane. no last name included).