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Before you start uploading, make sure your photos are the right size and shape for the format you plan to print.

To print successfully, your image file should match the photo print format you choose. If you forget about this you'll usually still get an acceptable print, but it may not come out exactly as you expected.

The 'aspect ratio' of an image is the ratio between its width and height. A standard 6x4 inch print has an aspect ratio of 3:2. Most standard digital image sizes, though, have a 4:3 aspect ratio. So the picture may not be the same shape as the paper.

Minilabs will scale your image to fit the format you choose, but they won't squeeze or squash the image to the same shape - that would make it look wrong. So there's a choice to be made. The whole photo can be printed, leaving a gap at each side, which looks a bit poor. Alternatively, the system can 'zoom and trim': the image is scaled to fill the paper, which means it'll go off the top and bottom, so these parts of the photo are trimmed off. The print has no awkward strips, but if your photo is a group portrait, tall people may end up headless.

In the software you use to upload your photos, most services allow you to crop your pictures manually. After you select a photo and choose a print size, a box shows which part of the photo will be printed, and you can move it around. This can be time-consuming for lots of photos, though.

To avoid the need to letterbox or trim, try to match the print format to your images. To help make this possible, most services now offer new 3:2 formats, sometimes called 'digital' as opposed to 'conventional'. These include 5x3.75 (12.5x9.5cm), 6x4.5 (15x 11.5cm) and 7.5x5 (19x12.5cm). Or you could go the other way and adjust your camera to shoot in 3:2 ratio. Professional cameras often do, and some compact models give you the option.

Resolution refers to the number of pixels that make up an image. Each pixel is a dot of colour. High-quality printing - good enough that the human eye won't notice the dots - requires around 300 dots across and down every inch, usually quoted as 300dpi (dots per inch). An image to be printed at 6x4 inches, for example, should have about 6x300=1800 pixels across and 4x300=1200 pixels down. That's 1800x1200=2,160,000 pixels altogether, or 2.16 megapixels. You can work out other sizes in the same way.

Reasonable print quality can be achieved with as few as 200 pixels per inch, but if you use a lower resolution than that - say, printing a basic camera phone image at 6x4, or a 2 megapixel image at 10x8 - quality will be visibly compromised. The image will look indistinct at first glance and blocky close up.

Printing a much larger image, such as from the latest 12 megapixel cameras, won't give you any visible quality improvement except at very large sizes, and will take far longer to upload and fill up the storage space you're allowed much faster. The service may not even accept files that big. You can downsize image files, before uploading them, using any image editing program. Remember there's little point in resizing low-resolution images larger, though, because quality won't improve.