The dark spot is where the light source intersects the surface. Your light source is too big and doesn't fit inside the lamp shade. Use smaller light source(4-5cm) and maybe more than one.

Use background image with grass texture and make sure you have selected "Background image visible in 3D view"

<r>Make material on back faces of your model invisible.<br/>
<br/>
<IMG src="https://www.mediafire.com/convkey/8889/3ytzydr2rk8d3tp6g.jpg"><s>[img]</s><URL url="https://www.mediafire.com/convkey/8889/3ytzydr2rk8d3tp6g.jpg"><LINK_TEXT text="https://www.mediafire.com/convkey/8889/ ... d3tp6g.jpg ...

Import 3d model, apply texture/color, 3d view > Export to OBJ format, import.

still, it doesn't look good on rendered photo

<r>I tested your case and this is my solution:
<LIST><s>[list]</s>
<LI><s>[*]</s>Select "Create walls" - double click on the grass,</LI>
<LI><s>[*]</s>delete grass,</LI>
<LI><s>[*]</s>select <U><s>[u]</s>"Create rooms" - double click<e>[/u]</e></U> somewhere inside these new walls,</LI>
<LI><s ...

Without downloading your file i can suggest: either draw "grass" room AROUND the house (NOT UNDER it), or raise the house level a little bit so it's no longer at the same elevation as grass level.

<r>Copy your project, open in winrar, extract Home.xml. Open Home.xml in text editor, find all lines with word "sash" and delete them, save and add to your project.<br/>
<IMG src="https://www.mediafire.com/convkey/6e4d/rik1zcpk674i46f6g.jpg"><s>[img]</s><URL url="https://www.mediafire.com/convkey ...

Try Select Hide Utility plug-in

<r>You should specify "dropOnTopElevation" (value 0 or -1). Create library of your model and edit PluginFurnitureCatalog.properties or edit Home.xml (part of sh3d file). Library(sh3f) and project (sh3d) are zip archives - use any zip tool to extract and add files. <br/>
<br/>
PluginFurnitureCatalog ...