It seems to be very common problem between new 4images users.
I uploaded a new template into /templates/ folder, then when I selected it in ACP (Admin Control Panel) - nothing changed
The answer is simple: you are trying switch to a new template in the wrong place.
It must be done in the
Settings not at
Templates* link in ACP
* Since 4images v1.7.2 that link was renamed to "Edit Templates"