How does Shakespeare depict summer's unpredictability in Sonnet 18?

In Sonnet XVIII Shakespeare uses the concept of sublunary, or earthly, corruption to demonstrate that summer weather is unpredictable.

In his Petrarchan sonnet, Shakespeare invokes the sense of harmony of the classical form and order demanded by this particular sonnet form. But, to expect such order in the universe is not possible because of sublunary corruption: Summer is but one season and then changes; sometimes it is too hot, or "rough winds" may disturb the beauty of nature. At any rate, this lovely season can end in destruction: "And every fair from fair sometime declines."

Since the elements of nature are transitory, verse is then appropriated as the form for the perpetuation of the speaker's love. Indeed, beauty will last forever in verse. In this way, the beloved's "eternal summer" will not wither or fade, and the beloved will remain as fair as she is at the time of the composition of this sonnet. The final couplet summarizes this eternalness:

So long as men can breath, or eyes can see,
So long lives this, and this gives life to thee. 

What image in lines 3-8 of Sonnet 18 shows that summer weather is unpredictable?

In Shakespeare's time there was not really such a season as spring as we know it now, so summer was joyfully celebrated much earlier - in March!, probably in the time we now know as spring. So to the people then, early May would have been known as summertime. So I would choose the "rough winds shake the darling buds of May" as my image in showing that 'summer' is unpredictable. After all, in England, early May comes straight after windy cool April so the weather is really not that much different yet - although when the sun does come out, it can be hot. (But not for long sadly!) Shakespeare himself associating the word 'May' with summertime as the word 'summer's day' appear too.

Concerning your question about summer's unpredictability in Shakespeare's Sonnet 18, I'll quote the lines for you and put the imagery that answers your question in bold:

Rough windsdo shake the darling buds of May,
And summer's lease hath all too short a date;
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd,
And every fair from fair sometime declines,...

Rough winds, heat, overcast skies, and fair days that lose their fairness make summer weather unpredictable.  I believe any one of the images answer your question, but if I had to choose just one, I would probably choose "too hot the eye of heaven shines," since it is more concrete than the other images, and specifically refers to summer, as opposed to the wind in May, which could be interpreted as referring to spring.

Incidentally, the only line above that doesn't contain imagery uses metaphor, comparing the length of the season to a rental lease. 

There are at least couple of different images that the author uses to show that summer weather is unpredictable in the lines that you cite.

First, he uses the image of buds being shaken from a tree.

After that comes the main image -- sort of a personification of the sun.  So I guess the image there is of the sun as a person.

We are told that his complexion is often dimmed.  This means that the weather is sometimes cloudy.  At the same time, however, his eye sometimes shines too hot.  So that really shows that the weather is unpredictable.