Yeah, I had tried everything it seemed, so I was at the stage where I was ready to be just told how to do it
I got it in the end with this piece of code on an updatepictures form
Code:
<input type="image" src="img/new/ico_del.gif" alt="delete photo" name="pictures[$pictureinfo[pictureid]][delete]" id="cb_delete_pic$pictureinfo[pictureid]" value="1" tabindex="1" />