You can change an image anytime you like and then re-upload your survey, but you need to think about the consequences of the change.
First of all, your data is safe. It will not be deleted. Just the programming files will be modified.
If it is just a simple changing of an image on a text page, that should be okay.
What you can also do is note down when the change occurred as you may want to see any differences in results with Image1 versus Image2.
I also like to include a new variable flag to indicate the change. On the text page where the image is to be changed, you can place this SSI Script ...
Now go into your Pass-In Fields and define "ChangeImageFlag" as a whole number.
So when your data has ChangeImageFlag=1, you know they are the resp[ondents who have seen Image2 (the new image) in your survey.