I would disagree with two of the Elemental representations you found.
Salt is not used to represent Water, but rather can be used to represent the Element of Earth. However, a sea shell could be used to represent Water, as could a blue candle.
Flower petals and leaves are generally used to represent the Element of Air rather than Earth as you have suggested. Representations for Earth could be a bowl of dirt or salt, a stone or crystal, or even a green candle. It is also associated with and can be represented by the pentacle.
Air can indeed be represented by incense or a feather. It can also be represented by the athame, flower petals or leaves, of a white or yellow candle.
Fire can be represented by the wand. It can, and often is, represented by a red candle. I have also used a piece of volcanic basalt to represent Fire.
Goddess representations - A statue of the Goddess, a white or silver candle, a cowrie shell, a stone with a hole through it, the Chalice
God representations - A statue of the God, a white or gold candle, a piece of stag's antler, animal teeth, the Athame.
If you belong to a specific Wiccan Tradition there will be a certain way that one is supposed to set up your altar and certain representation for the Elements and the God and Goddess that you should use. If you are a solitary then you can choose what speaks to you of the Elements and of the Gods.
There are also different kinds of altars that may have different items placed upon them.
For instance, you might have an altar to honor a specific Deity. In that case you might want to use altar cloths that represent the colors associated with that Deity, objects sacred to that Deity, an offering bowl, etc. As an example you can see a picture of my Deity altar that is primarily for Sekhmet, but also includes my other patrons at http://www.spellsofmagic.com/view_pic.html?profile=66900&file=%2fpics%2f00%2f66900%2fmini_1366909524.jpg
Or you might be using your altar to celebrate a specific Sabbat such as this one my coven used for a Mabon celebration: http://www.spellsofmagic.com/view_pic.html?profile=66900&file=%2fpics%2f00%2f66900%2fmini_1384708510.jpg
And of course you might have a working altar where you are doing magical workings. Such an altar would include the usual Elemental and Deity representation, but would also include the tools and ingredients necessary to your spellwork.
There's an excellent book that discusses various types of altars called "Circles, Groves, and Sanctuaries" by Dan and Pauline Campanelli.
You can find some more information about Wiccan altars at: http://www.spellsofmagic.com/read_post.html?post=506217