If you used the image from img folder with absolute path or a folder outside the project directory, this would not work on a deployed app.
Better way would be to create a image collection, add the logo to that image collection and then use the image widget to display the image.
If you cannot access the page in Mendix i.e., login page.
Then you can logo image in img folder under project directory and give a relative path starting inside theme folder
And do note that you need to empty your cache from your browser or try a different browser. Had the same issue yesterday with an environment. Only after a cache cleaning the new image showed.