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Image must be same format as thumbnail
Regarding images that are uploaded through the Textpattern admin interface (tabs: content > images), please note that you cannot mix two formats: the large image and the thumbnail must be the same format, because the database only stores information about the large image and sets a boolean value if it has a thumbnail or not.
So you can have different images with different formats.. but each thumbnail must be the same format as its corresponding main image. So, jpg with jpg and gif with gif.
(additional thanks to Johan)