If I use global stylesheet ( [TTStyleSheet setGlobalStyleSheet:[[[StyleSheet alloc] init] autorelease]]; ) than I don't see any thumbnails in TTThumbsViewController.
I specify these styles for TTThumbsViewController:
- (UIColor *)navigationBarTintColor;
- (UIColor *)backgroundColor;
- (UIColor *)tablePlainBackgroundColor;
- (UIColor *)tableHeaderTintColor;
- (UIColor *)thumbnailBackgroundColor;
If I set thumbnailBackgroundColor to grayColor I see gray rectangles in the controller but no downloaded images.
If I comment use of global stylesheet I see thumbnails correctly.
I tried use stylesheet in TTControl demo app and TTThumbsViewController behave same as in my app.
Here is screenshot from TTControl demo app:

I hope I am not doing something wrong. Can anybody have some idea why this is happening ? Thanks. Bye.
Please see the following Tutorial to help you with implementing
TTStyleSheetproperly:Three20 Stylesheets iPhone Tutorial