alec wrote:

Resolution (as in dots-per-inch) is irrelevant for images used on
web-pages. The only thing that counts is the number of pixels.

Huh, I thought that lower resolution would make the file size smaller so
web images would load faster.  No?
Images on screens (which is how we view web images) are shown at 72-90 pixels/in.  72 used to be a pretty standard monitor resolution, but now 90pixels/in (or more) is common in this day of lcds.  Assuming 72 pixels/in  a 216x216 pixel image would look 3" across.  The same image printed on a 300-2400dpi printer would be .72" (for 300dpi) to .09"  (for 2400dpi).

When he said that resolution was irrelevant he meant that a 900 pixels wide image doesn't really know it's resolution.  It's just 900 pixels across which would take 12.5" to display on a 72dpi screen, 10" on a 90dpi screen, and only 3" to display on a 300dpi printer.  The truth is that the image doesn't have a resolution, the display device does.  (This is only mostly true as I'll explain in the last paragraph.)

The truth is that Gimp displays to you using your screen resolution, so if you originally created an image at 72dpi and 216x216 pixels and another image at 4800dpi and 216x216 pixels, Gimp will display them the same.  While they look the same, if you look at image properties with <ALT><RETURN> or set the disply units to inches in the bottom border and move around the image, you can see the difference.  One is reported as 3" across and the other as .045" across.  The resolution is used by Gimp to translate to inches and inch derived units for you.

If you go into image/resize, and only change the dpi, Gimp will report to you that the image is a different size in inches, but the pixels are not in any way changed. 

Various image file types like jpg and png store resolution and Gimp does store that for you.  Devices are supposed to scale the images so that on their display resolution they will appear the same size as in the images native resolution.  Some devices/software actually do this.   If you print something and it comes out the wrong size, some times you can open it in Gimp, change the resolution and resave.  The only change will be in the stored resolution, the file's image data is completely unchanged, yet it will now print a different size!

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