Depens a lot on what program are you using. But lets get to that later.
- Try to texture a bit the objects. Even Paint wich is the most basic program ever, has a spray that can give you a bit texture.
- Projected shadows. This is the most basic thing to give deept to an image, In the example one object is now clearly behind another.
- Use external resources like textures. I googled Old wood texture.
- Add elements. For example bolts to the upper thing, etc.
But one very important thing is that you shoud use a more advanced program than Paint… if that is what you are using.
Using for example Gimp gives you the posibility to work in layers.
Layers is a realy, realy, realy big diference when working on a digital file.
Now you can work on an element and work on an element behind it without making a mess.