The fastest way to boost the image of your site is with photographs. Specifically, your own photographs.
There are a lot of images out there available with a quick right-click Save As... but if you head down that road you're likely to run into copyright violations faster than you can say "download dancing GIF."
Unless you're lucky enough to own a digital camera you'll have to scan your own images in order to get photos onto your site.
Most likely the scanner's default is set for color photographs but double check to be sure. Always in color unless the image is black and white and don't choose an option like "line art" or "halftone".
The trickiest scanning decision is choosing your resolution. In almost every case you'll want your web-based images to be the same size or smaller than their real life photo counterparts so in almost every case you can feel comfortable setting your input resolution to 100% or 72 dpi. However, if you plan to alter the image in a graphics program by all means try out the higher settings.
Keep in mind that bigger isn't always necessarily better: since you'll be saving these images in compressed formats you won't want to start with megapixel levels of precision or the image might show signs of degradation down at web-friendly size.
Once you've transformed your Kodak moments into timeless binary format sit back and take a closer look at them. What messages do they convey? Should you crop out needless background details to focus the viewer's attention on the salient details? Are they too bright or too murky?
A lot of mediocre images can be instantly improved just by adjusting the brightness or contrast with tools bundled into every modern graphics package.
Take some time to explore your software options and see what effects you can generate. I am particularly fond of colorizing photos so that they look like old-fashioned sepia toned pictures. This can actually serve as a neat trick for adding color images to a site without bulking up on large kilobyte full-color files.
If you want to play around with more exotic tools like implosion or crazy swirls and you don't have access to an expensive program like Adobe Photoshop you can always try out an online webware product like GIMP which not only allows you to transform your images like the pros but can compress the final products into more efficient smaller packages.
Compressing images, in case you're wondering, is not some complicated extra step you need to take nor does it make the actual image smaller. A digital image is made up of different colored pixels neatly arranged row after endless row.
Instead of accounting for each and every pixel individually, image file formats employ ingenious schemes to track large groups of colors at once reducing the amount of coded information that needs to be saved while preserving all the detail of the original image. All you need to do is choose a format from a drop-down file menu when saving your pictures.
The two most popular image formats are JPEG ("jay-peg") and GIF ("GIF" with a hard "g" as in "good"). 99% of the time you'll want to save your photographs as JPEGs which are designed to cope with complex color patterns that blend and fade.
Because GIF is better handling images with large areas of identical colors that format is ideal for text images and logos or maps with simple lines. So unless your photograph is a picture of beach ball throw up against a clear blue sky save your images as JPEGs.