new image size not saving

im trying to change the ppi of an image that is suppose to be at 1200 but reads as 72.  I open the image in Mac and resolutions reads 72 even thought the originator said its suppose to be 1200. I then open it in PS, go to image, size and change to 1200ppi & resample then save as.  Yet image is still at 72. How do I get art to stay & save at 1200?3grdhkkjkkkkk

I have an image that that someone has sent to me and is suppose to 1200ppi. It's a .gif.  When I open in Mac, resolution reads 72. I need it to be 1200. I then open it in PS, go to image size, change it to 1200 and resample and saved as. Yet when I open it, it still reads 72.  How do I get it to stay at 1200 ppi??