Several illusion spells would be useful to keep with the caster so as not to waste them on only one encounter. Notably, the -image line of spells (silent image, minor image, major image, persistent image, programmed image, and permanent image) have a range of Long (400 feet + 40 feet per Caster Level), and an effect line of either 4 10 cubes or a 20 foot cube each adding a 10 foot cube per caster level, (Shapable)
The range of certain other spells, notably the detect- line of spells, like detect thoughts (Range 60 ft.), is assumed to move with the caster as its origin.
Regarding Range, Aiming and Duration, the SRD says
RANGE: A spell’s range indicates how far from you it can reach, as defined in the Range entry of the spell description. A spell’s range is the maximum distance from you that the spell’s effect can occur, as well as the maximum distance at which you can designate the spell’s point of origin. If any portion of the spell’s area would extend beyond this range, that area is wasted.
and
Effect: Some spells create or summon things rather than affecting things that are already present. You must designate the location where these things are to appear, either by seeing it or defining it. Range determines how far away an effect can appear, but if the effect is mobile it can move regardless of the spell’s range.
and finally
Concentration: The spell lasts as long as you concentrate on it. Concentrating to maintain a spell is a standard action that does not provoke attacks of opportunity. Anything that could break your concentration when casting a spell can also break your concentration while you’re maintaining one, causing the spell to end. You can’t cast a spell while concentrating on another one. Sometimes a spell lasts for a short time after you cease concentrating.
[...] Effects [...]: [...] If the spell creates an effect, the effect lasts for the duration. The effect might move or remain still. Such an effect can be destroyed prior to when its duration ends[...]
The spell effect of major image, as well as that of permanent image can, with concentration, move throughout the range of the spell, but is the range of the spell measured from the caster's current position, or from his original position when first casting the spell?
My character, Gnorman the Gne'er-do-well, an 11th level Gnome Illusionist was a profligate, yet also a skinflint. He wanted to make some illusory friends to both share his travels and aid in his defense. He finally grit his teeth and put the powdered jade in with the wool and cast a permanent image of an enormous flock of about seventy flying winged monkeys, complete with little red coats and blue fez hats, with green tassels. Each monkey had his own musical instrument, and they played symphonically as they flew; it sounded distinctly Wagnerian.
Gnorman then concentrated in order to move the flock 800' in the direction he intended to travel and stopped them there, they settled down at attention and waited stoically, nary moving a muscle as he approached them. Gnorman was double moving, not thinking about the spell, as he hustled past them, and moved another 800' away from the grounded aerial monkey orchestra.
Then Gnorman slowed down to his 20' pace, and once again began concentrating on the winged primate musicians, willing them to rise, once more into the air, and fly alongside him, continuing on his journey with his new entourage.
Can he?
Long question short, can a permanent image be made to move with the caster, essentially forever, with minimal effort?