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I am using photoshop to crop Youtube preview images like this one, which is a .jpg file, the url being "http://img.youtube.com/vi/g-LvhmaHEWA/0.jpg"

enter image description here

Originally it is 480*360 and about 25k on disk, what I do is change the canva size to 480*270 so that the black bar will be cropped out. But after saving the file as .jpg file too, its size turns to 100+k. I changed the image quality to the lowest and still it is about 40k.

Orignal downloaded image (25k)

enter image description here

Trying to save with "save for web Legacy" (why the original is 380k???)

enter image description here

I guess the key problem is the original size of the picture shows as 380K, why???

------Update-------

This answer of this question states that for photographs it is more in line with intuition. But in my case, the image becomes about 2 times the original size, it is total counter-intuition. Moreover, I've tried open a 21K youtube preview image in PS, and directly save it without any editting, with image quality being 0, the file becomes 30+k for all "format options". So I think the rational behind image becoming larger is different from what that question answers.

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The "original size" you are seeing isn't a JPEG, it'll be either a PSD or a TIFF. Photoshop isn't looking at the original file, it's looking at the image that's currently open in the application, and that's an uncompressed image.

The reason why you're seeing a larger file size when you try to save a new JPEG version is that the JPEG you're working with is badly compressed with a lot of artifacts. Photoshop (or, rather, the JPEG compression algorithm) sees those artifacts as image data and will try to preserve them. Basically, the crappy picture seems to be more complicated than the "original original" clearer picture was, so as counterintuitive as it may seem, you'd wind up with a smaller file in the end if you started out with a much larger one.

There is a Photoshop tool (and there are better plugins) that can "de-JPEG" the image to a degree, but the image is small and the artifacting here is pretty severe, so it's highly doubtful that you'd see much improvement - cleaning up the artifacts automatically would probably destroy the image. If you put some work into it, you can probably kill some of the "ringing" and blockiness manually, but you really have to watch that you don't turn it into a painting as you're doing so. The smaller an image is, the harder it is to keep it looking like a photograph.

  • I try to find the Photoshop tool you mentioned and insteadly find this website pixlr.com/editor, after cropping and saving the .jpg in a 40 quality, the image file size becomes a little smaller(20K). What technique are they using? Besides, if the image file becoming larger is due to some meta data(if that is what "artifact" means), why then when I screen capture the 480*270 part of the picture from a browser, and the captrued JPG file is still terribly larger? Does screen capturing also capture the "artifacts"? – shenkwen May 5 '16 at 12:29
  • It's not metadata. The artifacts are all those little squiggly lines you see around the edges of things and the blocky areas of solid colour. They're not part of the original image; they're a result of the original high JPEG compression. But they're part of the picture now. If you can do a screen capture of the high-quality video, you should have a much better, cleaner image to work with, and that will let you make the file size smaller. The Photoshop tool is Filter->Noise->Remove JPEG artifacts. – Stan Rogers May 6 '16 at 2:10

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