By default, Showkase uses the first image in the gallery as the representing thumbnail on the gallery index page.
You can choose any gallery image you like for the gallery index page thumbnail as follows:
(1) Edit the gallery page (on the 'Pages' tab).
(2) Go to the 'Images' tab.
(3) Select the image you want as your representing image in the 'Index' column.
(4) Click 'Save' and then 'Publish'.
Please note that if you rearrange the order of images in the gallery, then it will be the original first image (before reordering) that will be used as the representing image (unless you change it as above).
Showkase has no caching functionality built-in at all. Changes are made instantly and you should see them immediately after publishing.
If you do not see changes immediately after making them, then try clearing your browser's cache before reloading your web page (or view your web page in a different browser that has not been used to view the web page before). This should let you know whether browser caching is at the root of your problem.
If this does not help, then it would appear that there is some server-side caching going on that you might need to investigate further.
I hope these notes help.