When the poet writes in Sonnet 24 of finding "where your true image pictured lies," he focuses on a meaning of "true" in the sense of genuine as opposed to counterfeit. The young man's beauty is often cast as a shape or appearance. Paintings, pictures, visual images, forms, shadows, reflected shapes, and perspective — all of these allude to the impression that the youth's true image is, in fact, a mirage.
Note that the poet's elaborately stylized writing in this sonnet — the first eight lines are an extended metaphor of the poet as a painting onto which the youth's image is painted — is the very kind of writing the poet criticizes elsewhere. But the poet is defining what he sees as he discovers its power over him, almost as if love itself is the creation of a need in oneself where none previously existed.
The poet's gazing at the youth in adoration impresses the image so indelibly upon his heart that the result becomes a private fantasy, totally self-induced, which allows the poet to possess the youth's beauty. In effect, their two personalities are combined. By praising the youth, the poet flatters himself as well. However, the concluding couplet raises doubts once again as to how authentic the poet's depictions of the young man are, and also of how important the young man's physical appearance is as a reflection of his inner feelings and personality. "Yet eyes this cunning want to grace their art" means "But because my eyes see so much beauty in the young man, I want to show his physical appearance most beautifully;" and "They draw but what they see, know not the heart" exposes the limits of the poet's truly getting to know the young man in any way other than through physical attraction. Ultimately, the poet's sonnets are limited in how much of the young man they can portray. The youth is presented as only a surface reality, with no depth of character.