> 1. I have built an experiment but I would like to use only the spacebar as command.
If you want your <trial> elements to accept the spacebar as input, you need to define it as the applicable /validresponse.
/validresponse = (" ") or /validresponse = (57)
If you are talking about instruction pages, you need to make the proper settings in the <instruct> element, i.e., define keyboard as /inputdevice and the spacebar as /nextkey.
> 2. I would like to add pictures in the first page where I explain to the participants about the test.
Assuming you are talking about instruction <page> elements, those cannot contain images. You either need to create HTML files containing your instructions and images and then display them using <htmlpage> instruction pages.
Alternatively use standard <trial>, <text> and <picture> elements to display your instructions.
In general I would strongly recommend completing the tutorials included in the Inquisit documentation before moving on. Those will help you acquire the basics of Inquisit syntax / how to set up simple procedures with it.