The image should be stored and processed using techniques that capture and retain as much quality — colour, tone, and and original pixel detail — as possible. Conversion to the limited resolution, and limited colour gamut required by the web, or a projector or computer monitor, should be the very last thing you do to an image.
Two rules — whether you're preparing for print or digital display:
Rule 1: Don’t edit original image files
If you make an error in editing an image and it becomes unusable you can always retrace your editing steps on another copy of the original file. You can never recreate the original file if you have edited the original pixels, especially if you also broke Rule 2!
Only ever edit a copy of your original.
Rule 2: Don’t edit JPEG files
The enemy of straightforward quality printing is JPEG because, by its nature, JPEG approximates colour and tone values in individual pixels, in order to achieve a smaller — compressed — image file size.
When an image is saved in JPEG, information is thrown away. Irretrievably. To make matters worse, every time a JPEG file is opened, edited and re-saved, further compression and further loss of information takes place. If you broke Rule 1 your original is lost forever.
JPEG suffers from two other disadvantages:
Edit and save files in an uncompressed format, such as TIFF.
If possible you should edit your file in a large colour space (ProPhoto or Adobe RGB). Do not convert your file to sRGB unless it is really necessary. If it is necessary, convert to sRGB only at the last moment when the conversion process will have the maximum amount of colour information to work with.
Image data decisions
The options you have depend on whether your camera is capable of saving RAW files or not, and what kind of photo editing software you have.
1: My camera allows me to save RAW, and I have Photoshop, or Lightroom, or …
Shoot in RAW, and use your computer’s software to handle RAW conversion, and colour and tonal corrections.
Good RAW converters, such as Lightroom, Aperture, Digital Photo Professional, Nikon Capture and Adobe Camera Raw (ACR), (which is part of Photoshop and Photoshop Elements) enable a host of RAW photo adjustments, including cropping, straightening, colour and tonal adjustments, spot and red-eye removal, sharpening and noise reduction.
Most of those aplications also have functions that enable you to print with precise colour control and resampling to the exact resolution required by your printer.
If you need to display a file on the web, or send it electronically for someone to view on an unknown monitor, it is probably safest to assume the lowest common denominator of digital colour systems. Lightroom, Aperture, Digital Photo Professional, Photoshop, and Photoshop Elements, all enable you to output a JPEG file with exact resolution, and have good routines for converting the colour space of your image to sRGB while retaining the maximum amount of colour data if you ever need to do this.
2: My camera allows me to save RAW, but I want to do more editing than is possible with RAW files.
Shoot in RAW, and use Lightroom, or ACR or the software that came with your camera to handle RAW conversion and initial colour and tonal adjustments then export (or ‘Save As’) the image as a TIFF file.
A TIFF file can be further edited, printed, and later converted to any other file format, but it is not compressed when it is saved. Image data is not deliberately thrown away in processing TIFFs.
When you save as TIFF you will likely get the option to ‘embed' a colour profile. Choose ProPhoto RGB or Adobe RGB to preserve as much colour information as possible. Do not choose sRGB if you have a choice.
Now open and edit the TIFF file if you want to do any further editing. If you need to display your image on the web or a digital projector or send to an unknown monitor you can 'save as', or export your TIFF file in JPEG format with sRGB just as for RAW files.
3: My camera only saves JPEGs
All digital camera sensors actually collect RAW image data. However, not all cameras give you the option of saving the RAW file; instead, a dedicated computer inside the camera converts the image to JPEG before saving it to your memory card.
If your camera saves only JPEGs then:
Now open and edit the TIFF file if you want to do any further editing. That way, the original JPEG will remain without further compression as a sort of Master back-up file in your pictures directory, and any edits you do on the TIFF copy of the file will not cause unnecessary image degradation.
When you are happy with your edits, save the TIFF file.