If you are wanting to create your own web graphics ( I suggest this as only you know what you have in mind) all you really need is a good program, a little creativity, and a lot of patience.

My personal favorites are Adobe Photoshop and Compact Draw which can be found at:
http://www.compactdraw.com/compactdraw/index.htm it is also quite afordable

Both programs are the most user friendly and allow you the most options with out needing a four year bachelors degree in graphic design to operate.

Of course any program you are comfortable with is the best choice. There are also a variety of free software prgorams you can try by going to: http://freewarehome.com

Well, images that represent you or your product are appropriate. Such as logos or product imagess. Or perhaps screenshots that help to explain a process.

It is perfectly acceptable to use images as design elements such as bars, bullets, and navigational arrows. But use these with a lot of self-control and only if they compliment your web site.

Viewing other web sites is a good way to find out what not to do. There are ebooks of all levels. Good ones and bad ones. To decide which is the best way, shop around, be a viewer yourself, and decide as a viewer before you become a designer.

The most important tip is to remember to use images only as they are appropriate. Never for just the sake of using them. They will only promote confusion with in your ebook. Those dancing babies are neat, but probably don’t belong on your web site. Also drop the annoying flashing gif animations that are hard on the eyes and will be distracting to your readers.

If you wish to use banners on your web sites, be careful in your selection and place them at the bottom or top of your site away from your main content.