When considering images here are some of the effects of the wrong settings used.
The rule of thumb is 2x the lpi ruling for the screening – Most Litho printing is 300 ppi (Pixels to the inch)
If an image is over sharpened halo’s and artefacts are visible, if you over use unsharp masking an edge will be visable. If you over use overall sharpening you will increase the grain in the image. The scale of the image will change the amount of sharpening you need. ( A stamp would be different to a poster).
Compression is very hard to demonstrate on a computer but there are basically two types of compression “Lossy” and “Lossless”
Lossy compression removes some data and so if you compress and re-save the file over and over again you will gradually degrade the image. Lossless as the name suggests is less efficient and the files are bigger but the compression does not degrade the image.
It is worth remembering that although you can make the image look very bright and “punchy” on screen but if that is done on an RGB monitor in the RGB colourspace you may well be disappointed when the final image is printed. RGB monitors can display far more colours than a printing press can print and so make that if colour is critical that your monitor is calibrated to CMYK colourspace and that you are working in CMYK mode.
When adjusting images consider the highlights as this is critical for the contrast of the image. The point at which the highlight turns from pure white paper to containing some level of detail can change the image significantly. If you set this level wrong light areas will either be washed out or too dark. The points at which you set your highlight, mid-tone and shadow will help with the image reproduction.
When you look at an image on screen it is very difficult to see if there is a colour cast because your eyes adjust. by comparing several different adjustments you are much more able to asses the right cast for the set of images – pull all the images that you intend to use together onto screen and adjust so that they are similar. Remember print has a tendency to go warmer (more magenta) so if in doubt go a little bluer.
A useful tool in many photgraphic packages like photoshop is gamut warning – this will highlight any areas of the image that do not fit the intended colourspace. You can see the green area in the top right is showing that this may change colour when printed CMYK. This is not a critical area and can therefore be disregarded, however if it was in the foreground it may be necessary to adjust the settings.
We have all probably taken a photograph in doors and had a very warm or green cast across the image. This is caused by the temperature of the light source. Different lights (Tungsten, Fluorescent, daylight) have very different colours and where possible it is necessary to adjust the camera so that it is correcting for the colour of the light. Many cameras auto white balance but when photographing in mixed light sources it is important to review the white balance settings.
Once you have these key settings right you should get much better images in your printed product.