Hi Jean,
I need to make one thing clear. You can't alter any of the items that are installed in the game. You can only add to them.
Custom floors, for instance, are just like carpets effectively. They cover up the default floor, they don't change it.
If you want any other things, walls, pillars, other objects, you have to make then in a 3d program, save them as 3ds files and then use them as custom models. That is the only way to do it.
Also, nothing can move. The only animated things are the installed things. You can only make objects that don't move. That goes for texture files too. It would be nice to have moving things but you would have to pester Patrick for that.
There are several formats you can use for texture files. Bmp, tga, jpg, but gifs don't work. We usually use jpg as the file is smaller, and generally the quality is good enough.
As for the jpg file names, you can use the names that come with the files or, if there is only one file, change it to match the model#.
It's the model file that has the information about the textures, and it knows what belongs where, but it will also accept the model#.jpg name. The difference is that it will only use one file if you name it model#, but you can have many files if you use the original names. I will say at this point though that no texture file name can be longer than 8 characters or it won't show up. So 'gingerbread.jpg' wouldn't work but 'gngrbrd.jpg' would.
Keep asking questions as you feel the need. You're probably more confused than most because you have done work for The Sims and this is so different. I've not had experience of anything other than Wonderland so it seems natural to me.
I'm sorry that I have to keep telling you that things are wrong, but you seem to have a zip file in a zip file here. Plus you only have one texture for the gazebo and it needs three. You can only name the textures model#.jpg if there is one of them. If there are more you should leave the original name on them.
Thanks to Kymmie.