In the many "OMG - Detect Magic/Create Water/every cantrip is the devil!" thread, all have failed to take into account or otherwise ignored a single, interesting bit of rules pertaining to Detect Magic.
3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).
From what the spell states, unless the magic aura is being generated specifically from an item or from/by a creature, there is no way to actually discern what the spell is. There is just a magical aura of some order of magnitude. Or that's how I am reading it. A spell without a target (like the Image line) would only ever be identified as a Magic Aura of a strength relative to its caster level. You could never deduce what kind of magic it was. Similarly, if someone shot up a room with a fireball and you walked past and used detect magic, you could discern magic happened there of a particular strength due to lingering aura, but you would never know what the magic was.
Chew on that thought.
This forum is for rules questions, not rules debates. (Stop laughing! I'm serious!)
What's the actual question here?
What? Rules questions? This is absurd! I could make it a question but I guess it would be better to go to General then.
This is more like an answer. The questions are legion.
Can detect magic reveal invisible creatures?
Can detect magic reveal that a creature has been charmed?
Can detect magic reveal magic traps?
Can detect magic reveal illusions?
These are all common questions, and Carty's post sheds some relevant light on each of them.
Not exactly Rules Questions Forum material, but it could be found under "A:" in an FAQ in the future.