It doesn't get much simpler than:
From the description for Invisible Spell:
Those with detect magic, see invisibility, or true seeing spells or effects active at the time of the casting will see whatever visual manifestations typically accompany the spell
And, from the description for Silent Image:
This spell creates the visual illusion of an object, creature, or force, as visualized by you. The illusion does not create sound, smell, texture, or temperature. You can move the image within the limits of the size of the effect.
However, any characters WILL see the material component of your spell during the casting process along with any verbal and gesture components required (the material consumed during casting, thank you @KRyan) regardless of Invisible Spell. In this case, a small piece of fleece won't be easily missed but it's something to be kept in mind.
Note that this feat has no bearing on any components required to cast the enhanced spell, so the spell's source might still be apparent, depending on the situation, despite its effects being unseen.