Photostore runs on PHP with the smarty template engine.

To modify the store you must have some knowledge in editing files. Depending on how much you want to do you will need to know CSS (color, fonts, layouts, etc..), HTLM (layouts, etc..), and PHP (addition of or changes to features).

If you have no knowledge of any of those then you will have a hard time to customize it (or anything else web based).

Smarty template engine is nice as it allows a way to seperate PHP code from HTML/javascript. This makes it easier to modify layouts, colors, etc.. It allows the store to have "themes" some people call them templates, all the same. Those themes can then be selected from the store manager manager -> look & feel area. Right now there is only one theme (default) that comes in multiple colors. We hope to add new themes in the future. We would also like to see 3rd parties build themes as well like what started to happen with version 3.

The themes contain a set of CSS files that control all the colors, fonts, sizes of boxes, etc..
It also contains a set of TPL (template) files for smarty engine to use. These TPL files should be considered the same as HTML files since they have HTML as well as smarty code in them to load the data from the store.
Then there is a set of images and icons which are used for that theme.

As for integration with wordpress, joomla, and such, we don't know if we will ever go that route, since our product is designed to run as a standalone site or in conjunction with an existing site but in it's own directory like for example (www.---my--domain---.com/store/). We can't rule this possibility completely out, but as of now we just don't get enough request for this, maybe one or two people every now and then ask about it.

I'm hoping that as the new version (4) matures and becomes more stable, there will be a point where the updates can be installed and the template can be left alone (or require minimal tweaking). At this point, major features are being added and the templates have to change.

This is the number one reason I have not upgraded my store yet. Maybe Jeff and weigh in on this - this is what makes third party template sales impossible, and I think there's a large market for selling ktools templates.