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I have tried everything I can think of, but when I reduce resolution of graphics, keeping the same dimensions, it has no effect on the size of the file.

This makes no sense. I prepare graphics for the web and so I often want to drop some weight by lowering the resolution of an image from, say, 300 ppi to 72 ppi. It would seem, intuitively, that this would lower the size of the file. Yet, when I do it, the file size stays the same.

Is there something I'm missing? I've tried with resampling on and off, to no avail.

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You would have to have Resample Image checked 'on' to accomplish what you are trying to do. - can't immediately think why that would not work. –  Rsiel Apr 9 at 14:31
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If you're making images for the web, you should be ignoring ppi settings entirely. All you should pay attention to are the pixel dimensions. FYI, just changing the ppi setting does nothing to change the amount of data in the image. It just changes how big it will be if you print from DTP software. –  DA01 Apr 9 at 14:55
    
its also questionable wether the ppi or dpi setting is even used in printing, most likely the image goes to page layout somewhere. And then the dpi is just some startingpoint size that somebody may alter. –  joojaa Apr 9 at 16:31
    
@joojaa it's used by some software to figure out how big to print it. For example, some page layout programs will read the PPI setting when importing an image and size it to the page based on that by default. –  DA01 Apr 9 at 18:25
    
@DA01 yes but its by no means authoritative. So you can not really rely on it. Its more like a tool for estimating size untill you actually go to the next stage. –  joojaa Apr 10 at 3:24

2 Answers 2

If you're using a set amount of pixels, let's say 500 x 500, it won't matter whether you're using 300 ppi or 72 ppi because the amount pixels in the image would still be 500.

If you wan't to lower the file size of your image you will either need to scale down the image or save the image in a lower quality format:

enter image description here

Or use Save for Web as this can give you a finer degree of tweaking with your image quality as you save:

enter image description here

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The key portion of the answer for the OP is the first sentence, ignoring the definition of resolution. From a file size perspective, DPI is stored in probably somewhere between 2 and 4 bytes and changing the DPI value doesn't alter the number of bytes used. An RGB pixel is a minimum of 3 bytes per pixel. So removing even one pixel reduces file size. Then compression, then profit. –  horatio Apr 9 at 14:37

Raster images (which is what Photoshop works on) are stored as pixels. The more pixels, the larger the file size.

The ppi (pixels per inch) affects how the image appears on paper. By increasing this number, the pixels get smaller when printed. However, changing this number does not affect the number of pixels. If you have a 1920 x 1080 pixel image, the file size will be the same whether you have the image set to 72ppi or 300ppi. The only difference is how big the pixels will be when printed.

To reduce the file size by changing the resolution, you need to change the number of pixels. By keeping the width:height ratio the same, your image will have the same aspect ratio when resized. So you could change a 1920 x 1080 image to 1280 x 720, and the number of pixels (and file size) would be only about 44% of the original.

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One confusion for people new to this is that software often automagically resamples (adds/removes pixels) when they edit the dpi value. When they change 100dpi to 300dpi, they fail to notice the pixel dimensions tripling. –  horatio Apr 9 at 16:33
    
"The more pixels, the larger the file size." is wrong. File size is about compressed raster data almost always, and 1000x2000 px of white pixels compress to smaller file size than 100x200 of any photo. But even when we are talking about versions of the same picture, a scaled up version could compress to the same file size as the smaller one, with the same kind of compression. But it depends how well the compression algorithm handles the kind of image raster involved. –  Volker Siegel Apr 9 at 23:41
    
@VolkerSiegel all other things being equal, the more pixels, the larger the file size. You are correct, of course, that compression can make larger images actually smaller file wise than small images, but I think that's only going to confuse the OP. :) –  DA01 Apr 10 at 3:53
    
True, this will not help solve the OP's question, and could even add confusion. But now, he is warned regarding this comment, so I can still insist, without causing harm! :) From the theory side, there is no reason that size depends on pixel count - it should only depend on the "amount of information" in the image. If you make an photo larger by repeating each pixel four times, there is a very small information change compared to a photo. So no reason the file needs to be larger - may be some bytes larger, but the compression could handle the bigger image better, saving more bytes. –  Volker Siegel Apr 10 at 4:17
    
Actually, I tried. The effect I described at the end of the last comment occured: I created plain white images in GIMP and exported to png with same settings (max compression). Second image is 100 ps larger, and has smaller file size: 100x100-white.png: 293 byte, 101x100-white.png: 279 byte. –  Volker Siegel Apr 10 at 4:18

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