How can you define Brand Integrity? Does it aid in enhancing the Brand image ?

I agree that brand integrity definitely supports enhancing the image - in case the integrity is implemented for brand attributes that are in line with the desired brand image. 

Brand integrity - to me - means that the brand in represented consistently in all touch points where customers meet the brand and that all interactions support the desired brand image and positioning. For example, if I had a brand who is positioned to be cheaper than the competition, no frills, then it must be cheaper than the competitors in all outlets and cities. Furthermore, such a brand will not stretch to making a line extension for a premium product - this would require a different brand.

In short, integrity comes from consistency and definitely supports the selected image. 

It clearly helps in enhancing the brand image. To stay true to your brand philosophy while achieving your marketing objectives, is the best thing to happen for a brand. It means your idea, positioning and your brand philosophy has worked in the market. Very few brands have achieved this in complete sense. Volvo Trucks is a good example.