This is not a Game Design question (this was in the wrong forum). This question has been moved to a more appropriate forum.

Look at this realistic satellite map from google maps:

Now look at this map which has been mini-mapped by them:

You can equate this to minimap vs rendering. Rendering is meant to be real: Proportions are real , and it's very detailed (noisy) . Your eyes love noise (details). Without it things don't look real.
In contrast, the minimap is meant to be readable. There is no noise (the grass is solid green). They added borders to important features (ex: street blocks don't really have a dark grey border around them in real life. ). The proportions are wrong: If you zoom in/out to a certain degree, street widths will stay the same. There is no lighting or shadows either!

Doing some of this (proportiions) is really hard, and I don't suggest that you do it (at first).

Start with reducing noise on your map images:

1. Remove the textures from everything
2. Remove the shading (the green rectangles are shaded)

Then, if you have the patience, add borders to everything (this is moderate effort, skip it if steps 1 & 2 look like they are enough)

@Haziq

How about using simpler textures (for instance solid colours) for your minimap lets say light yellow for walkable area's and black for walls. As an example I took your map and made a quick and dirty mock up:

I passed your minimap through an edge detect filter in GIMP.

Note: While you can do this in real time: a minimap's background does not tend to change in real time. So you can use a static image that you enhanced in Photoshop beforehand.

@Haziq no, you're minimap is already fine as is. I was just suggesting borders. There is not requirement for anything :-)