create jpegs 300dpi size
how do I create jpegs size 300dpi
my os high sierra on mac, with just photos program on it
iMac, macOS 10.13
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300 dpi is not a size, it is a resolution (300 dots per inch)
Size/DPI is irrelevant until you want to print the image. If you want to set the parameters, you need to export the image to a folder, and then open with preview, and use tools>adjust size.
If you want a bit of a primer on the relationship between DPI, pixel dimensions, and image size, see the post here:
https://www.creativebloq.com/graphic-design/what-is-dpi-image-resolution-71515673