When adding a smilie through the "Add multiple smilies" feature in the ACP if the image happens to be missing and the dimensions are not entered or are removed for the smilie the user is presented with an error:

[phpBB Debug] PHP Notice: in file /includes/acp/acp_icons.php on line 401: getimagesize(./../images/smilies/icon_arrow.gif) [function.getimagesize]: failed to open stream: No such file or directory

along with an sql error (attached) which is a result from not having the image available to recalculate the dimensions. I suppose a check for the existence of the file prior to calling getimagesize would be ideal and simply producing an "No smilies were added" error message such as when you forget to check the "add" box would be enough.