Legacy Maemo 5 Documentation/Human Interface Guidelines/Icons
Icons
Many applications share similar tasks that can can be represented by a generally known icon. For example, several applications have "Previous", "Next" or "Back", "Forward" actions that can represented by arrow icons. For these kind of actions applications should not ship their own icons, they should use the system icons instead (known as stock icons). An alternative is to ship any desired icons but using the common task ones only when there is no equivalence in the system.
In a device with a touch screen, since there is no mouse, there will be no tooltips, hence choosing an icon when there is no text label in the widget involves an even more important decision than in a desktop computer. If the user doesn't readily understand what's the action assigned to a button with only an icon, he or she will need to read any help document about the application but is likely to press that button in order to check what's its function (which might not result in what the user wants). If an icon doesn't seem clear enough for the task it represents, then don't use it, use a text label instead. Text labels generally give a better explanation to a user. Apart from the mentioned common tasks, the only way where an icon might be preferable to a text label is when there's no space for the text.
For general guidelines on the design of good icons, please read the GNOME HIG Icons section.