renaming uploaded files

Hi php developers: I have created a website for a house renting company with an administration tool. Using this tool, managers can add, modify and delete data in a mysql database. They can also upload some pictures of the houses they rent. The way i do this is by taking them to the list of houses, making them select one and next they go to a form. In this form they upload files to a certain folder in the server. What i would like to do is auto rename the file with a name composed by IDNUMBER + NUMBER OF PICTURE (ex. if ID of that house is 345 image name would be 34501.gif, 34502.gif, 34503.gif,...) I think you got the idea. Any help would be greatly appreciated. thx in advance