To add a texture pack, simply add a folder here in the AVWW/RuntimeData/TexturePacks folder.  

The name of the texture pack in the game will be the name of the folder.  So if you make a folder called BobsTextures, that's what it will show up as in the game.

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HOW TO REPLACE A TEXTURE USING THE TEXTURE PACK:

Inside your texture pack folder, simply duplicate the contents of the Images folder that you want to replace.  Don't bother moving over images that you aren't doing custom versions of -- the game will first look into the active texture pack folder and then will look into the base images folder if nothing was found.

So, for instance, if you want to replace the keyboard cursor (normally located at AVWW/RuntimeData/Images/Misc/KeyboardCursor.png) you would add AVWW/RuntimeData/TexturePacks/BobsTextures/Images/Misc/KeyboardCursor.png.

That would alter the keyboard cursor, but no other parts of the game.  You can do that for as many or as few images as you like.

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HOW TO REPLACE A PARTICLE EFFECT USING THE TEXTURE PACK:

Inside your texture pack folder, make a Configuration folder.  Then make a Particles.xml file that is a copy of the main Particles.xml file in AVWW/RuntimeData/Configuration/Particles.xml.

Simply make <g pattern="Fireball"></g> entries for particle effects that you wish to change.  Don't worry about trying to do them all -- just do the ones you're actually making changes to.  The others will load from the main Particles.xml file just fine, but your custom particle definitions will selectively override whatever you wish.

Did you know?  When you have the game open, you can hit Ctrl+F4 to reload the Particles.xml files at will.  That way you can make a small tweak, hit that key combo, and then test your change immediately without having to restart the application.

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TIPS:

Generally speaking, you need to keep the images to the same size and dimensions as the original.  Any wildly different image sizes won't look at all correct in-game, especially when it comes to sprite dictionaries (animations).

That said, when it comes to things in the Backdrops and Ground folders, those can be set to pretty much whatever size since they are repeating tiled graphics.