According to the PHP documentation, the default value for the memory_limit directive (for the current version of PHP) is 128M but it looks like many web hosts reduce this.
Even if you were to create a gallery (including thumbnails) outside Showkase and then import it (via the 'Site -> Import' functionality), the gallery would then become a genuine Showkase page and you would likely run into the same problem again.
At the moment, the only way around this problem (other than increasing the memory_limit value) would be to manually embed your Juicebox galleries in 'Basic' Showkase pages using the baseUrl method of embedding documented here (although this defeats the purpose of being able to create and manage galleries within the Showkase interface).
In any case, the developers will certainly investigate this issue further to see if anything can be done to prevent this from happening.