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I've been given an image (I'm guessing straight from a camera) and it's currently at at 240 pixels per inch. It's been a while since I did much Photoshop; I have three questions for two requirements:

  1. Back in the day the web used 72dpi images because that's all screens worked at. Is 72dpi still the most appropriate resolution for the web?
  2. I need to use the image in a phone application. I know phone devices have higher DPIs than 72 but do you need to feed them a high-DPI image to begin with, or will a massive 72dpi image work properly?

BONUS QUESTION

  1. How do I resize an image from 240dpi to 72dpi while maintaining detail? i.e. I don't want to go from a 240px * 240px at 240dpi to 72px * 72px at 72dpi.
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If your image is 240px * 240px at 240dpi, and you change the resolution in photoshop to 72dpi, the image is still 240px * 240px and the quality isn't changed. It is just a reference for if the image is to be printed.

If you're using an image onscreen you can ignore the dpi, it will only ever display the number of pixels.

I've found the best way to export an image is if you have Photoshop, you can use File > Save For Web, and it has all sorts of presets for the main web graphic formats: png, gif and jpg, you can experiment with these and display 2-up or 4-up to compare different formats or compare with the original.

For use on the phone app, you only really need to know the resolution of the phone, ie the number of pixels across by the number of pixels high. For the best display on that phone, set the pixels across and the pixels high to as close to that as possible before exporting

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