the texture paint features in blender are very poor. You’d be much better off using another app for 3d painting, or even not doing 3d painting and doing your textures in photoshop
make sure the extension is correct, and good policy, is to save the file before you save the texture painted image, then don’t save again after. that way blender doesn’t get confused with which image it has loaded. but check to make sure that you have the output extension correctly, because it saves in whatever format you have set in render buttons, not the format of the loaded image.