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Well, I don't know if it's the place to put this but, I want to know if I can use the Normals texture on the JME.

And other question is, can I make games with the graphics quality of a Xbox 360 game? (I know it's difficult, I just want to know if it's possible).

Can I make a game that the player can open a music or image from the pc and the game use it?

And, a last question, how many people can play on online gameplay, and to play online, it need's to use a browser?

Thanks.

I want to know if I can use the Normals texture on the JME.

Yes, you can use normal maps in JME. There are some examples of this in the jmetest package.

can I make games with the graphics quality of a Xbox 360 game?

Well the smart answer would be it depends on the 360 game. The short answer would be no.

Can I make a game that the player can open a music or image from the pc and the game use it?

Yes.

how many people can play on online gameplay

As many as your code can handle - the network code is not within the scope of JME.

and to play online, it need's to use a browser

No, that is not necessary.
can I make games with the graphics quality of a Xbox 360 game? (I know it's difficult, I just want to know if it's possible).

Currently jME is mostly targeted toward low and mid-end video cards, although it does provide some shader compatibility, it's only added as an extra feature, rather than integrated into the engine core. Recently however, a new branch was started, "jME3" which aims to address these issues. jME3 provides a flexible shader architecture, material files, content management, and other features which allow development of games with comparable graphics to Xbox 360, and other modern consoles.

jME3 has a test that demonstrates normal/specular mapping on a model:
http://code.google.com/p/jmonkeyengine/source/browse/branches/mf_jme3test/src/g3dtest/fx/TestBumpModel.java
can I make games with the graphics quality of a Xbox 360 game? (I know it's difficult, I just want to know if it's possible).

i don't see what should be the problem, except the hardware you are throwing it at. Like Mokomo_Fan said, it's essentially made for low to mid-end but the engine should be capable to achieve console game graphics - also depends on the coders and gfx artists of course :)