Always use a text box to display the image and configure the path of the image in Text section, and select the Image option in the Representation list box.
Whenever there is a change in the image just replace the existing image with the new image in the same path. Qlikview will automatically pickup the new image. The default method won't do this, it will save the image in the Qlikview file itself.
So I always suggest you to use the Text Object.
Hope this helps you.