Go to the 'View' menu, drop down to and select 'Snap To...'; follow the little right arrow and drop down to 'Document Bounds'.  If this has  a checkmark in front, your tools will 'snap' to the images' edges if you come close to them. Uncheck it to loose that snapping attitude. I have it unchecked all the time...

This is the right way to make the tools 'Snap' or 'Unsnap' to your image's edges...

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ