What happens when a person "imagines" how food could taste like just from looking at it? What processes are triggered by imagining the taste of food (let's assume it's sealed so that its smell doesn't reach the test subject's nose) only by looking at it, based on memories of food with similar color, shape, texture and size?

If the food seems to be good, usually saliva is generated, but what other things happen?
As Josh Gitlin said, imagining something activates more or less the same parts of the brain as actually experiencing it. Behrmann (2000) is a good introduction to this in the visual domain, though that article is a bit outdated now. There is also a lot of evidence that representations of meanings (semantic memory) involve a distributed network of perceptual regions (e.g., Binder &amp; Desai, 2011; also Barsalou, 2008) and there are some computational models that try to capture this (e.g., Rogers et al., 2004). Putting this evidence together suggests that the sight of food (visual input) would spread activation through the semantic network and would activate the other perceptual, emotional, and linguistic associations (taste, smell, food-related memories, etc.). Salivation is one possible outcome of the activation of taste and smell representations; but I think it is also reasonable to expect activation of memories of eating related foods, etc.

Dana Small is one person who has done some very interesting studies on the neuroscience of taste and smell perception, including top-down effects like anticipation (e.g., Small et al., 2008).

References

Barsalou, L. W. (2008). Grounded cognition. Annual Review of Psychology, 59, 617–645.

Behrmann, M. (2000). The mind’s eye mapped onto the brain's matter. Current Directions in Psychological Science, 9(2), 50–54.

Binder, J. R., &amp; Desai, R. H. (2011). The neurobiology of semantic memory. Trends in Cognitive Sciences, 15(11), 527–536. doi:10.1016/j.tics.2011.10.001

Rogers, T. T., Lambon Ralph, M. A., Garrard, P., Bozeat, S., McClelland, J. L., Hodges, J. R., &amp; Patterson, K. E. (2004). Structure and Deterioration of Semantic Memory: A Neuropsychological and Computational Investigation. Psychological Review, 111(1), 205–235.

Small, D. M., Veldhuizen, M. G., Felsted, J., Mak, Y. E., &amp; McGlone, F. (2008). Separable Substrates for Anticipatory and Consummatory Food Chemosensation. Neuron, 57(5), 786–797.
http://dx.doi.org/10.1016/j.neuron.2008.01.021