Images used well can reinforce meaning and direct the user. When not used well they add confusion and contribute to visual clutter.
Unless the meaning is very clear, or space extremely limited an icon should have a label. There are very few universally recognisable icons. If the meaning or intent is not improved by the addition of the icon, just use the word and a regular bullet or arrow.
An icon should be recognised in silhouette, instantly and understood. If the function is a common one (eg. Print) the icon should be entirely predictable. The weight and style of an icon should be balanced with the typeface used for buttons and labels.
Type in Helvetica Neue with icons of balanced weights & size
Arrows are the clearest shape we can use. Big or small their intention is clear. We use them in three ways:
Avoid the use of Museo for type on buttons, and use very subtle depth and shading.
Small amounts of reversed (white) text is OK on buttons, but always use on a darker coloured background that passes AA contrast tests.
Buttons should be clear, bold and very easy to read.
Switches and checkboxes should show the current state, and reflect that in colour
Buttons that link through to pages can use a chevron arrow.
Actions that work in place do not need an icon, unless it adds considerable meaning.
If in doubt, use a linked, bold text label instead.
These symbols are widely used enough not to need a label
Money Advice Service specific symbols may need labels
...and tool or app specific symbols definitely do