I assume you have a form that has a "browse" button (the typical upload text box)? if not, place one in the form. (seel below)
i can give you the code for the SQL but i presume (as your building the site) that you could probably implement what i say.
First of all the form:
use something simple like:
<form enctype="multipart/form-data" action="WHEREVER.PHP
<input type="hidden" name="MAX_FILE_SIZE" value="100000
" /><!--edit that default 100kb-->
Picture <input name="uploadedfile" type="file" /><br />
<input type="submit" value="Upload my image" />
thats from a source i used at http://www.tizag.com/phpT/fileupload.php
, the rest of the PHP code can be found there, its REALLY easy to implement
Say your databaseTable was called "Item" and within that you have several fields such as price etc etc, here make sure you have a field called "PictureURL" or something simular.
From here you can use both those codes together and have it so the value in "pictureURL" is defaultdir/<uploaded file>.gif where <uploaded file> is the value in the user submitted form. (location of the image)
Sorry if its not too clear, that site might be able to help you a bit more than i can, im not very good at explaining things