According to Russel's core affect framework an affective state can be decomposed into valence, arousal and dominance. How would you define these three terms (e.g. when looking at pictures)? I would define valence as the pleasure (positive / negative) and arousal as the activation level. But regarding dominance I'm absolutely unclear what this is...
These concepts (valence, arousal and dominance) are fairly standardized in the emotional investigation.
What do you mean by valence? Although at first the emotions could and were classified as positive and negative or as pleasurable and unpleasant both the relationships between the emotions themselves due to their nature (evolution characteristics, etc.) (they are not part of a continuum in which they are placed at the extremes but every emotion it functions independently) as the relationship with the person who experiences them make it pass from the pleasant or unpleasant classification to the concept of valence.
With arousal refers to the old classification as intensity of the stimulus although there are more in a hundred psychosomatic aspects.
Dominance, tension and kinetics are added to these concepts. In particular dominance would be in relation to the concepts of control and again this concept is related to psychosomatic aspects.
You have a document that goes deeper: http://www.levenswerken.eu/flash/pleasure_arousal_dominance.pdf