There are some basic rules for designing graphics that have been around since the stone age:

a) Use the rule of thirds.
b) Arrange on diagonals.
c) Create a central focus.
d) Background is background and foreground is foreground.
e) Form follows function.

In addition, there is one rule that overrides all of the other rules, but only sometimes:
f) Rules are made to be broken

Use this last rule only when the other rules fail to give you the result you want. This doesn’t mean you should fall into the trap of “Ooh, this isn’t working – I’ll break the rules to fix it.” The rules cover 98% of all graphic design cases. The time to break the rules is when you know that the rules aren’t sufficient to solve whatever creative problem is facing you.

a) Rule of thirds
Place important elements of the design on the one-third dividing lines of the image. Think 1/3s of the horizontal and vertical, not half-way through.

b) Diagonals
A diagonal in an image is created by any elements that line up along a slanted / diagonal line ie: parallel diagonals reinforce one another and give an overall form.

d) Backgrounds / Foregrounds
Establishing the proper relationship between the foreground and background is to do what you must to make the background less noticeable, and the foreground more noticeable. Don’t confuse, overload and dazzle the viewer.

e) Form follows Function
Web graphics must generate a message, identify a hyperlink, help the user navigate to another page etc. As Frank Lloyd Wright quoted, 'form follows function' ie: to be readable is more important than a stylish font.