Start off by running File Explorer, then open the Properties of the C: drive and run Disk Clean-up. The first pass cleans the current user data, but when it finishes there is an option to do the same for the system. This should free some space. You can select which areas are cleared, although I normally select all the areas when I run it.
If you have a separate D: or other logical disk, then check if you can move things like Documents, Music, Pictures and/or Videos from C: to some other logical disk. If you change the locations of these folders, Windows can move the data for you.
A disk image is the actual raw partition of the logical disk (C:). If C: is full, you either have to remove some information from C: or enlarge the size of the partition to add extra free space.
The ultimate answer is to replace the physical drive with a larger capacity drive and increase the size of any logical drives then clone the existing logical drives onto the larger drives on the new physical drive. Although there are many tools freely available to help with doing this, although you might require a degree of expert assistance to do this properly.