Changing Image Sizes

If we want to display larger images (as for example the Seasons theme does for featured items on the homes page), whats the best way to ensure an appropriately sized image is being used, as opposed to scaling up a thumbnail?

How would you override this just for a section on the home page?

Failing that, how can you increase the size of images everywhere?

The themes are usually scaling down thumbnails by providing a CSS maximum width and/or height, so the constraints and overriding is taking place just in the theme's CSS.

The actual pixel dimensions of the thumbnails and fullsize images get set by the site administrator in Appearance Settings, but the themes almost always apply some kind of CSS constraint to make sure large thumbnails or fullsize images don't break the design.

Sounds like the appearance settings should be able to solve our problem, thanks. I'm not seeing any immediate changes however. Is there some caching, or does this only apply to newly added images?

The size limits only get applied to newly-added files (though you can re-generate the old ones at a different size using the Derivative Images plugin.