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Image resizing is pretty straight forward. The following tutorial and specifications are all for Adobe Photoshop, but the only differences if you are using another editing program will be found in the menu system - the ideas behing image resizing will be the same.
First of all you need to decide what you are using the photo for. If you are going to be uploading it to a website or online photo album, a relatively small image size is all thats required, usually no bigger than 800 x 600. A resolution of only 72dpi is needed, dpi referring to Dots Per Inch and is a printing term that relates to the number of dots of colour required to print an image. Typical CRT monitors have 72 pixels per inch so 72dpi is all thats needed to display an image on screen at its correct size and quality.
For print, dpi is more important as a higher resolution is required to make sure the colours and quality are printed correctly. Since most printers only have 3 or 4 colours, more dots are required per inch to print the correct colours. Generally printed images should have a dpi of 300, and its recommended that dimensions are entered as centimetres or inches rather than pixels.
Photoshop (and most others) will rescale the dimensions of an image if the resolution is changed to compensate for the added or lost dpi, but not the other way around, so its quite easy to leave an image destined for the web at 300 dpi. The dimensions will still be correct but the file size will be a lot larger.
How to resize images
Image resizing is one of the simplest editing functions available, to open the Image Size dialogue box just select "Image" from the menu bar and choose "Image Size..." or hit Alt-Control-I (on PC) to get it. The screenshot below shows where it is:
If you are using Photoshop Elements, the menu option is also in the "Image" menu, but more obviously located within a "resize" sub-menu.
The screenshot below shows the Image Size dialogue box:
The settings here above are typical settings for on-screen use: 800x600 dimensions in pixels, and 72dpi. The dropdown boxes by the Width and Height boxes are used to enter sizes in other measurements, like cm, mm and inches, but as its for on-screen its best to work in pixels.
For print, the most important thing is to set the resolution to 300dpi, so that the quality of the final thing is high. Also for print, its best to set the image size using the "Document size..." settings, where they can be added in metric or imperial measurements (and a few others). Using these will change the Image size settings, and will choose the correct number of pixels in both directions for the photoshop file. Here the image size is set to 6x4 inches which is standard for photos.
There are a few other options in this dialogue box worth noting. The "Constrain Proportions" checkbox is used to make sure the ratio of the image stays the same. If not checked you could change the width of the image and the height will not change to match - skewing the image. This could be useful in some cases, but for photography it isn't. Constrain Proportions will automatically change the width if you change the height, and vice versa. If you use Photoshop styles, the "Scale Styles" checkbox will do them too.
Finally the "Resample Image" dropdown is used for manually controlling how Photoshop rescales images (how it chooses the colour of pixels that need to be put in when it enlarges the image). The default - Bicubic - is usually the best option, and this will be covered in more detail in a later tutorial.
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