With the RockSolid template it is more a settings issue than a general problem.
By default the resizing range in the templates configuration is set from 768 to 1140 px.
Below 768px all containers will be moved one below the other BUT the template stops to resize.
That means, for example, the header image becomes full sized and need 768px horizontal space with the result that you will see scrollbars on a sub-768px screen.
So the current solution is simple:
Click on "extensions->template manager". Select the "RockSolid" template from the list and hit the "edit" button.
On the "Options" tab enter a much smaller value into the "Select min width in px" input field. For example 480px should be a good value. So the images will not stop resizing on 768px screens but on below 480 screens. Small enough for most mobile devices.
BUT: Currently I work on the RockSolid template to improve the mobile experience (which isn´t perfect...). Including font readability, streamlining the layout adding more navigation elements and changing the default size range. Important for small but high resolution screens like the iPhone4+ retina display... I´am looking forward to release the new version today (maybe tomorrow morning...)
Its now a semi fluid layout from 480 - 1140px (can be ajusted in the templates config) and a main breakingpoint at 980px. On devices with this or a lower resolution (all mobiles, including iPhone4 plus smaller or lower resolution tablets) is shows the "non-desktop" version with bigger and bolder texts, a vertical layout and navigation etc.
On devices with a higher resolution (like an iPad with retina display or an average desktop computer) it shows the normal version.
That fixed the problem that the template uses its normal version on iPhones4/5 or other high resolution but small screen devices (you have to zoom and scroll to read the text).