There is some difference between the style of Tennyson and Keats in their use of imagery. First, though, the literary definition of imagery is sometimes vague and confusing.There follows a definition.
According to Dr. Kip Wheeler imagery encompasses (1) "metal pictures" inspired by the written text; (2) sensory perception, whether written in figurative language or literal language, and whether abstract or concrete and whether visual (sight), tactile (touch), auditory (sound), olfactory (smell), gustatory (taste), kinesthetic (motion), or thermal (heat and cold); (3) literary techniques of metaphor, simile, allusion. Much is encompasses by the one term "imagery," and if this is unrealized, it can be very confusing when asked to identify "imagery."
Some similarities between Tennyson's and Keats' use of imagery in poems like Tennyson's "The Lady of Shalott" and Keats' "Isabella: or, the Pot of Basil" and "To Autumn" are that they both use realistic and concrete as well as metaphoric and abstract imagery. In describing nature in "Shalott" and "Autumn," Tennyson and Keats both use realistic imagery (Tennyson: "on either side the river lie"; Keats: "With fruit the vines that round the thatch-eves run") and metaphoric imagery (Tennyson: "clothe the world and meet the sky"; Keats: "Close bosom-friend of the maturing sun").
While Tennyson's poetry may be more consistently evocative of emotion than Keats', which can also be contemplative as in "To Autumn," both poets use imagery to create emotionally involving poems as is true for "Shalott" and "Isabella." One way Tennyson evokes emotion is by combining visual imagery with sensory imagery related to character reaction. For instance, in "Shalott" the Lady's verbal reaction carries sensory imagery evocative of emotion ("I am half sick of shadows..."):
"Or when the moon was overhead,
Came two young lovers lately wed; 70
'I am half sick of shadows,' said
The Lady of Shalott."
In "Isabella," Keats relies more on allusion and sensory imagery descriptive of character reaction. For instance, in "Isabella" we are told sensory details of Isabella's reaction ("She withers, like a palm") and the allusions and metaphors surrounding her reaction reveals it in symbolic diction ("Sound mournfully upon the winds:"):
"O Echo, Echo, from some sombre isle, 435
Unknown, Lethean, sigh to us—O sigh!
Spirits in grief, lift up your heads, and smile;
Lift up your heads, sweet Spirits, heavily,
And make a pale light in your cypress glooms,
Sound mournfully upon the winds and low; 445
For simple Isabel is soon to be
Among the dead: She withers, like a palm..."
One other point is that there is a subtle difference in their diction levels. Tennyson's diction tends to be of a higher, more formal, level (Tennyson: "The gemmy bridle glitter'd free, / Like to some branch of stars we see") while Keats' diction, though not low nor informal, can be less formal and more casual ("And touch the stubble-plains with rosy hue").