I'll answer concerning the Browning poems you are interested in, and let another editor compare them to Keats' imagery.
In "Porphyria's Lover," Browning is addressing the obsession the outwardly polite and prudish Victorian society had with sensational stories of horror and depravity. The themes of sex and violence and madness in this dramatic monologue speak to this obsession.
Browning turns the conventional presentation of these issues and themes, however, by making them seem natural and beautiful. Porphyria glides (line 6) in amidst the rain and the wind and shuts out the cold and the storm (line 7). She builds a fire that warms the cottage (line 9). She bares her shoulder (line 17) and lays her hair upon his cheek (line 19). These are all images that create beauty and comfort.
At a poignant (a feeling of specialness) moment, she totally gives herself to her lover, and he, trying to preserve the moment, strangles her with her own hair, painlessly, according to the speaker.
Browning forces readers to contemplate the relationship between sex and violence and power and complex madness, as well as beauty.
Browning uses imagery to a different effect in "My Last Duchess." The portrait of his murdered wife may be beautiful, as may his other works of art, but beauty is not the issue, and isn't what's meaningful to the Duke. His wife is now, in the present of the poem, in a perfect state. She is the perfect work of art, reflecting back on him. He kills her because she does not behave as a work of art. As a portrait, she does. The Duke is interested in how works or art reflect back on him and his 900-year-old name.
One of the key images in this dramatic monologue is his wife's countenance, her face, its depth and passion revealed in her glance as captured in the portrait (lines 7-21). But the image isn't important for its beauty, but for how it's interpreted and twisted by the Duke. Almost anything could have brought that "spot of joy" to her face, says the Duke. His wife had the audacity to be polite when anyone showed her "courtesy," according to the Duke. She was "too soon made glad" (line 22).
The images that follow, the daylight, cherries, the white mule, all reveal that which might make his wife smile. And that is no good.
The Duke is much happier now that she is truly a work of art and, therefore, behaves like one. Now she smiles only when he pulls back the curtain and allows someone to look at her--someone like his silent listener.