Answer:
Changing the resolution (horizontal resolution = image width, vertical = height, image resolution = width x height) is the same with resizing an image. Generally speaking you can resize your images or change their format:
- on various WEB sites (google for image converter online). Their advantage is that you do not have to pay anything, to install anything expect maybe for a java engine. Their disadvantages are limited image size, limited conversion formats and an average quality of the conversion results (most use predefined php or asp routines).
- using image editing/viewing programs. They can be free (GIMP, Irfanview, Google Picasa) or commercial (Photoshop). They allow you to precisely set the conversion options resulting in good conversion results. Their weakness is the complex interface for a relatively simple task and the more or less poor support for converting a lot of images in one operation.
- using dedicated image converters (free or paid). They are created for photo/image conversion and are (typically) simple to use. As the name says, these tools are also used to convert images from one format to another (for example from tif to jpeg or bitmap). Since these are tools shaped for this task, they (typically) have no weaknesses in image quality, easy to use, advanced conversion options, batch mode support, ... Some of them support RAW formats (a must for professional photographers) and maybe upload to various web photo album services (for example picasa WEB Albums). An example of an image converter supporting everything noted above is AZImage (see reference).