Whenever you show a group of related items, it's important to show 1) that each item is different and 2) that items share some commonality that places them in the same conceptual group. These contradictory goals are most easily achieved by having the individual items be noticeably similar, but visually unique.

Here are some buttons from a web page I designed. 

The unifying features are:

• All have the same size and shape and border
• All use the same colors (black, white, and gray with a blue border)

Yet within these constraints, each button is unique:

• The camel is shown in negative space -- it's what's not there that looks like a camel
• The snowflake has shades of gray
• The chameleon is defined in negative space, like the camel, but also has shading suggesting skin texture and three-dimensional detail
• The porpoise is also defined using negative space
• The palm tree uses shades of gray to suggest lighting effects
• The man was someone "whose presence in London could compete for attention with an eclipse" so having the face extend past the boundaries suggests someone larger than life

I'll explain how I (as a non-artist) created these images in a subsequent post. The color scheme or lack thereof was chosen purposely, as I'll explain in the next blog post on " Using Color Effectively."