It's very simple.
A very important thing you need to do is setting the form as "enctype="multipart/form-data"
Code:
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST" enctype="multipart/form-data">
other fields here
... then the upload form field
Upload Image: <input type="file" name="imgfile">
</form>
if you Google you get lots of samples:
php image upload - Google Search