Well, the pics you posted didnt work for me. But in any case, the bumper on my 90 Celica is not screwed on, it is clipped on. The main clips are on the side, there is also a row of plastic fasteners inside the grill area at the top. If its out of alignment its possible that it just got unclipped. This is what the car looks like without the bumper :
Im not sure what screw you are referring to, but the bumper is attached in the following areas
1. Clips on the sides, see the two plastic things in the pic
2. Row of plastic clips pressed upwards into the main metal bar - non weight bearing, i didnt replace mine after removing them
3. Underbody tray is screwed into the underside of the bumper - again, non weight bearing but can rattle if its not secured.
To unclip you need to first unscrew the underbody tray so that it doesnt interfere.Then wrap the end of a long screwdriver with tape so that it doesnt damage the soft surface of the plastic. Then just prise it off, use the pic to see hwere the clips are. Once unclipped you can reallign it and clip it back. Mine was only clipped on one clip when i first re-installed it after respraying, and it stayed on perfectly well - just crooked. So its possible that this is the culprit. Let me know if you need more pics.