Q.: What file resolution do I need for printing?
Traditionally, commercial printers will ask for 300 dpi (dots per inch) files. This can often be relaxed to 240 dpi for inkjet printing. But even 180 dpi or less will give you great large-format prints. On the other hand, sharp images with very fine detail will benefit when printed at 360, or even 600 dpi.

Q.: Do I need to size my image files to 72 dpi for my website?
This is a common misconception. Other figures floating around are 84 and 96 dpi for web-based images. Fact is that the Web doesn’t care about dpi (that’s only important to the printer). You only need to worry about the pixel dimensions. For example, a 600 x 400 pixel image will come up as a medium-sized image. A thumbnail image typically is around 100 x 100 pixels in size.

Q.: How large can I print my files?
The short answer is: any size you want.
You can take a tiny 20 KB file and get it printed to billboard-size. You need to up-sample your image to at least 150 dpi, otherwise it will have jagged edges.
The upsizing is the critical part. The software needs to interpolate new pixels, based on the existing information. Needless to say, you won’t get new detail that wasn’t there before in the original file. It pays to use a good interpolation ‘algorithm’ to get the best results. Most image editing software will have a facility to upsize a file.

Q.: What file format should I use?
There are dozens of image file formats, but you’ll probably just encounter the following:

JPEG …. This is the most common file format on the Web and is also used by all digital cameras. It is a ‘lossy’ compressed file format, which means that information is thrown away to reduce the file size. If you intend to edit an image repeatedly it is important that you save the file in an uncompressed form, for example as a TIFF file.
TIFF …. This is the standard file format used for saving an image in an uncompressed form. Like the JPEG format, TIFF files can be read by virtually any program that handles images.
PSD …. Every image editing program has its own file format and PSD is the native format of Photoshop. Use it to save images with layers and channels (selections).
GIF and PNG …. These are file formats you’re likely to find on the Web. GIF files are limited in their colour range and therefore not well suited for photographic images, but they allow for ‘animated’ images.
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Q.: Should I set my camera to JPEG or Raw?
Raw is another image file format. It wasn’t mentioned in the previous question because it is only used as an output format in some digital cameras. When set to JPEG, your camera will process the images according to your settings (image size, compression, sharpening etc.). On the other hand, Raw files contain all the details the image sensor collects. The files are bigger because there is no ‘lossy’ compression, however, the files need to be processed through a ‘Raw converter’. The advantage is that you have more image information which allows you to get potentially better quality images from your camera.
If your camera allows for Raw output you should use it for best image quality.
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Q.: What do I need to know about colour management? Should I use sRGB or switch to Adobe (1998) RGB?
Colour management is a very complex subject. Luckily, this is a topic that today can be safely ignored in most cases. Your digital camera and any image editing software are set to sRGB by default. The Internet works with sRGB, and so does the printing service you are using. There are other ‘colour spaces’, of which the Adobe (1998) RGB colour space is most common one. In theory, they can give you a wider spectrum of colours than the sRGB colour space.
Another aspect of colour management is that it ensures uniform colours across all your devices: camera, computer screen and printer. If the colours don’t match, then you need to make sure that you use the right ‘colour profile’ for your device (usually the printer profile is the weak one) and you need to ‘calibrate’ your monitor and printer.