Google's PageSpeed Insights tool can certainly be very helpful but it is also useful to remember that the results are just Google's ideal recommendations for a perfect score which, in a real-world scenario, might not be easy to achieve or make a great deal of difference depending on the web page in question.
Here are a few notes on some of the recommendations.
You could make some of your images smaller in filesize by reducing their quality slightly. This would address this recommendation in the report and should increase the speed score. However, I notice that the images for your homepage's gallery are around 200KiB each and I personally do not think that this is excessive (knowing that Juicebox preloads images as required and does not load all gallery images at once). You are certainly best positioned to know whether you'd want to decrease the filesize of your images. You might be perfectly happy with the filesize and quality of your current images, in which case, you won't need to worry too much about this recommendation.
You could try enabling gzip compression on your web server. Please see this web page for details.
Eliminate render-blocking JavaScript and CSS in above-the-fold content
Prioritize visible content
Your Juicebox gallery is above-the-fold and it is absolutely essential that the gallery's JavaScript and CSS files ('juicebox.js' and 'theme.css') are loaded as soon as possible so that the Juicebox gallery can be displayed. In fact, almost everything on your Showkase homepage is above-the-fold so all external resources need to be loaded before the above-the-fold will be rendered correctly. There's really nothing that can be deferred.
Knowing the content of your web page and what files are required for the above-the fold content to be rendered correctly, the two recommendations above can be put into context. They are not truly problems but necessities for the gallery (and, indeed, the layout of your web page) to be displayed correctly.
You could certainly set HTTP cache headers within an .htaccess file if you like. You could allow files to be cached by browsers rather than force browsers to always fetch files fresh from your web server (which is what I expect your web server currently does, being that this recommendation is in the 'should fix' section). Take a look at this web page for more information.
You could minify Showkase's CSS files (for example using an online CSS minifying tool) but I doubt that it would be a worthwhile venture. I genuinely think that the speed increase that it would give you would be minimal and it would have a couple of drawbacks: the CSS would no longer be easy to read (or, indeed, modify if necessary) and if or when you upgrade Showkase, all the CSS files would be overwritten with un-minified new versions and you'd need to repeat the process all over again.
Showkase already loads minified versions of the jQuery, Modernizr and Juicebox JavaScript files.
There is very little other JavaScript content that could be minified and if you tried to combine all the JavaScript into a single large file, you'd break Juicebox (and it would be difficult to do within Showkase).
I wouldn't worry too much about the tool's speed score. I would be more inclined to just load your website in a browser (which is what visitors to your website will be doing) and make sure that it loads within what you consider to be reasonable amount of time (and then perhaps address some issues if you really feel you need to). You'll likely have a good feel for whether you think your website's loading speed is acceptable or not. When I view your website in a browser, I reckon it loads pretty quickly. The tool just gives you some recommendations to maximize everything in an ideal scenario although, in many cases, not every recommendation is achievable. Don't get me wrong: the recommendations are useful but, even if they are achievable and increase your resulting speed score (by becoming green ticks instead of red crosses), I think the real-life speed increase in a Showkase site would barely be noticeable.
Showkase does not create hugely resource-heavy web pages. It loads only a few JavaScript and CSS files (I've seen some websites loading upwards of 50 JavaScript files and a similar number of CSS files) and, because of this, there is really no need to minify absolutely everything for the sake of a few milliseconds and at the expense of making the code very difficult to read and customize. Anything you do to reduce the number of recommendations in the report would likely be for little real-life gain.
Try entering a few other websites into the tool and you'll probably find similar speeds to that of your own website being reported.
I've just entered 'amazon.com' into the tool (as an example) and it scores similar speed scores to your own website. Being that Amazon probably want as many customers as possible, if the speed of their website was really an issue, I'm sure they would have addressed all the recommendations themselves.
At the end of the day, there are certainly a few things that you can do: check the filesize of your images, enable gzip compression on your server and allow browser caching. Anything else would be difficult to implement within Showkase.
However, I'd be happy that your website has a score of 100/100 for mobile friendliness and loads pretty quickly in real life.
I hope this goes some way to putting your mind at ease.
Edit:
On Google's own site, the full report is immediately available on-screen without needing to be emailed.
According to the report, minifying CSS on your homepage would save 9.6KiB and minifying JavaScript would save 4.2KiB. These are not huge savings that are going to make much of a difference.