There seem to be a lot of studies about the influence of self-image on our psychology, but I've been wondering: Is there research on what type of self-image (positive, negative) most people have? Do people in Western countries tend to have a positive or a negative self-image?
... on the balance, most people tend to view themselves positively.
Not much is clear about self-image. One thing that is clear is that self-image is not one thing. In the research literature, it can refer to a conglomerate of fairly independent / mildly correlated constructs, such as body image, self-schema, self-valuation, self-esteem, self-perceived physical attractiveness, self-worth, etc... That is to say, one can score positively on some of these measures and negatively on others, with little relationship between the scores.
Culture may indeed play a role as well:
More recent research investigating self-esteem in other countries suggests that illusory superiority depends on culture. Some studies indicate that East Asians tend to underestimate their own abilities in order to improve themselves and get along with others.
The uncertainty between positive and negative self-image is underscored by the existence of both an above-average-effect, and a worse-than-average effect. Each may refer to different aspects of self-image, but may even refer to the same aspects!
For example, while college students rate themselves as more likely than average to live to 70, they believe they are less likely than average to live to 100. People regard themselves as above average on easy tasks such as riding a bicycle but below average on difficult tasks like riding a unicycle.