The screenshot you have of the "cubes" in the FPSC environment look almost like the real thing Unfortunately, there may be an issue that needs to be dealt with.
The following are taken from the MineCraft Terms of Use page:
Quote: "You're free to do whatever you want with screenshots and videos of the game, but don't just rip art resources and pass them around, that's no fun."
Quote: "If you wish to make something pertaining to anything we've made we're humbled, but please make sure that it can't be interpreted as being official. Do not make commercial use of anything we've made."
The first quote is clear. The second quote is where the gray area for some lie. If you are making what is called "fan-based" media, then you can do it for yourself; but if you plan to distribute said media to a public forum (such as here) you may need to:
1. Obtain written permission from Mojang that they acknowledge and grant permission of your idea.
2. Contact TGC and provide them with the permission obtained and ask if you may provide this on their forum/site.
3. If steps 1 and 2 are done, then you would also need to provide said permission from both companies to the end user to protect everyone, considering that we all live around the globe and laws vary from country to country. This also protects not only TGC and Mojang, but you as the developer.
If you are being questioned on the work, then it has been interpreted as being official. If your pack contents look like the real thing or are
interpreted as the real thing, then you can't distribute it without the required permissions.
Based on your screenshot on your site, the textures look almost like the real thing, and therefore the
interpretation could be there for some.

Twitter: @NFoxMedia