Your galleries are SimpleViewer-Pro galleries and what you are describing is the Mobile Player version of SimpleViewer galleries.
SimpleViewer-Pro is predominantly a Flash image viewer with an HTML 5 fallback (known as the Mobile Player) for mobile devices and browsers which do not have Adobe Flash Player installed and enabled.
It sounds like the Mac browsers you are referring to do not have Adobe Flash Player installed and enabled and SimpleViewer is using the Mobile Player to display the galleries.
Please see the note regarding Universal Playback here.
Universal Playback automatically ensures that all users are able to see your gallery images, regardless of whether or not they have Flash-enabled devices or browsers.
However, please note that the Mobile Player supports fewer configuration options than the Flash version of the gallery. Here is a list of options supported by the Mobile Player. (The Mobile Player does not support Watermark Options or imageClickMode.)
Please also see these FAQs:
When I view my gallery I see a grid of thumbnails, not the layout I specified. Why?
The mobile player layout does not match the Flash player layout. Why not?
As you have Showkase-Pro, you could use Juicebox-Pro instead of SimpleViewer-Pro for your galleries.
Juicebox-Pro is a fully-featured HTML 5 image viewer that does not require Adobe Flash at all.
More information on the differences between SimpleViewer and Juicebox can be found in this SimpleViewer blog entry.
Juicebox-Pro supports imageClickMode (like SimpleViewer-Pro) but does not support dynamic watermarking.
However, you could create your Juicebox-Pro galleries with JuiceboxBuilder-Pro (the desktop application) which can watermark your images when they are resized for your galleries. Please see here for details.
You can then upload the gallery folders to your web server and import them into your Showkase site using Showkase's 'Site -> Import' functionality.
Once a gallery has been imported, it becomes a genuine Showkase page (just as if the gallery had been created in Showkase itself) and can be edited from within the Showkase interface if necessary.
(1) Upload the complete gallery folder to your web server using an FTP program. (I recommend uploading it to a location outside your Showkase directory.)
(2) Go to 'Site -> Import' in your Showkase site.
(3) For the 'Import path', enter the path to the directory containing the gallery folder (not the gallery folder itself).
(4) Click 'Scan' and choose the gallery folder (or folders - you can import several galleries at once if they are all in the same parent directory).
(5) Click 'Import' and 'Publish'.
The newly imported gallery page will have its own entry on Showkase's 'Pages' tab.
... and may change my galleries to all look like that (if I knew how)...
In SimpleViewer-Pro, you can force the Mobile Player to always be used (to display the grid of thumbnails), regardless of whether Flash Player is available or not, by deselecting the 'Use Flash' checkbox in the 'Standard Viewer Options' section.
In Juicebox-Pro, you can force Small Screen Mode (see here for more details) to always be used by setting 'Screen Mode' to SMALL in the 'General (Pro)' section.
Small Screen Mode is similar to the Mobile Player in SimpleViewer in that it displays thumbnails and images on separate pages (not together). (However, Small Screen Mode in Juicebox-Pro supports far more configuration options than the Mobile Player within SimpleViewer-Pro.)