Ponieważ bo:
Źródło (co za niespodzianka): http://www.flickr.com/help/photos/#18Flickr officially supports JPEGs, non-animated GIFs, and PNGs. You can also upload TIFFs and some other file types, but they will automatically be converted to and stored in JPEG format.
As you publish photos, they're compressed and resized by Flickr (if necessary) in the following sizes:
75x75 pixels
100 pixels (on the longest side)
240 pixels
500 pixels
Large (which will be 1024 pixels if it exceeds that length)
The original size (if you have a pro account)
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