Make the default image format for Qwen image 2.1 png #1559

Feature Idea

When attempting to generate images with transparent backgrounds with Qwen Image 2.1, they came out with a purple background, this seemed weird but then I realized it was because the outputs were all jpg. Setting swarm internal Image Format to PNG makes it work, but since the default is jpg, users need to override the swarm internal setting for image format to PNG, or you'll get purple background usually.

I suggest setting the default output format for Qwen Image 2.1 to PNG or WEBP.