Can Silent Image be used to make an illusory wall? I think it can, but there is an spell called Illusionary Wall which specifically does that. If I can use Silent Image to make an illusion of a wall, is there a compelling reason I'd ever use Illusory Wall instead?
You are correct that Silent Image can create a wall. Both spells belong to the Illusion school, with the "figment" subtype, and allow a will save to disbelieve the illusion. But each spell has its advantages and limitations.
Silent Image is a temporary, cheap, and flexible illusion.
Consumes a level 1 spell slot.
On the bard, magus, and wizard/sorcerer spell lists.
Range is Long (400 ft. + 40 ft./level).
The maximum size scales with the caster's caster level ("four 10-ft. cubes + one 10-ft. cube/level").
Requires concentration to maintain. A caster could have only one Silent Image active at a time.
If someone succeeds the will save to disbelieve the image, then they can partially see through it. According to the rules on illusion spells, it will appear translucent upon a successful will save, because the Silent Image is a figment.
A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.
Illusory Wall is a permanent and effective illusion of a wall or similar surface.
Consumes a level 4 spell slot.
On the wizard/sorcerer spell list only.
Range is Close (25 ft. + 5 ft./2 levels).
Illusion size is constant (1 ft. by 10 ft. by 10 ft).
It has permanent duration, where as a Silent Image requires concentration to maintain. With enough castings of Illusory Wall, you could eventually create a permanent illusory castle!
Unlike other figments, even if a creature succeeds the will save, the illusion is still effective. Someone may know the wall is an illusion, but they still cannot see what's on the other side!
Although the caster can see through his illusory wall, other creatures cannot, even if they succeed at their Will save (but they do learn that it is not real).
\$\begingroup\$ Deleted my answer in favor of your far more complete answer. \$\endgroup\$– MiatogOct 11, 2017 at 15:12
1\$\begingroup\$ @Miatog Sometimes other answers can be very helpful, since it may be your way of phrasing things that helps a user understand what's going on. And 2 answers is quite tolerable -- I wouldn't be saying quite this same thing if we had 10 different answers to this question. :) \$\endgroup\$ Oct 11, 2017 at 21:19