How can I make my visual analysis less descriptive and more analytical?

The key is to make sure that your visual analysis is integrated into your argument. You don't want to be writing several sentences of visual analysis without linking it back to your argument. You also don't have time to include information that does support your point, even if you are keen to show your knowledge. I'm going to give two examples and let's discuss the differences. Say we are trying to explain how gesture enhances narrative in Masaccio's 'Tribute Money' fresco... 

Ex 1. 'One example of how gesture is used for narrative can be seen in Masaccio's 'Tribute Money', a fresco in which a tax collector approaches Jesus and his disciples and asks for money. Jesus instructs Peter to get the money from a fishes mouth in the water and Peter is then able to pay the tax collector. The scene is broken down into three segments but we can follow the narrative through Masaccio’s use of gesture, which directs us from the first scene to the second scene. The first scene is Jesus dressed in pink and surrounded by his disciples and from his gesture we go to a second scene with Peter crouched by the river bed collecting money. Peter copies the gesture in the first scene.'

Ex 2. 'An example of how gesture opens up new narrative potential can be found in Masaccio’s ‘Tribute Money’, at the Brancacci Chapel, in which Jesus directs Peter to the water to collect tax money from a fish’s mouth and pay the tax collector. At the centre of the composition we can clearly make out Jesus, whose arm points towards the water, re-directing the viewer’s attention to the next scene in which Peter collects the money. This gesture is reinforced by Peter, who points in the same direction so that Jesus’ gesture becomes a core part of the work’s composition. In this case, use of gesture is allowing us to follow the narrative in the right order.'

In the second example we focus only on relevant info (including a bit more background eg Brancacci Chapel) and so we have more time to ensure our point is very clear. We don't merely describe what we can see in the painting, but we analyse the effect of a particular gesture on the viewer. Rather than merely mention that Peter is repeating the gesture, we use the word 'reinforce' to explain why the artist has chosen to do this, and then make reference to its effects on the overal composition. 

When doing visual analysis we can think - why has an artist chosen to do this? What effect does it have on the viewer? How does it affect the overall composition? and integrate these considerations into the visual analysis to keep it analytical.