How to open a dwg file and combine it with the image file on the drawing being drawn? I want to combine .dwg and .tif files on the screen and draw them like a combine .png file wihtout saving like a file.

Simplest way is to attach image background to a .dwg file. Image background can be stretched to fill entire viewport background, or you can use image tiling. If you need more exact control for raster positioning, you can attach raster image entity to a .dwg file, but you will need erase it before file saving if you don't like to save raster inside drawing. Alternatively, you can create addition OdGsView and attach transient raster image entity to it. In this case you will be able to control positioning of your raster image.