Hi Ray,
Following the solution recipe, I first changed the existing image field from "text" to hidden.
I then added the new file field, as directed. The code I ended up with is:
<tr>
<th>picture:</th>
<td><img src="<?php echo $row_WAATKmembers['picture']; ?>" alt="img" width="80" />
<input type="hidden" class="WAATKTextField" name="picture" id="picture" value="<?php echo(str_replace('"', '"', $row_WAATKmembers['picture'])); ?>" size="32" />
<input type="file" name="pictureFile" id="pictureFile" /></td>
</tr>
</table>
<div class="WAATKButtonRow">
<input type="image" hspace="0" vspace="0" border="0" name="Update" id="Update" value="Update" alt="Update" src="WA_SecurityAssist/images/Pacifica/Elemental_update.gif" />
<input name="UserID" type="hidden" id="WAATKUpdateRecordID" value="<?php echo(rawurlencode($row_WAATKmembers['UserID'])); ?>" />
</div>
</form>
