You can create sophisticated concept diagrams by the tikz package of Latex (PGF/TikZ is a set of languages to produce vector graphics from geometric description). Some example here: http://www.texample.net/tikz/examples/tag/diagrams/
On the other hand, 3D computer graphics can help you create cool scientific images. The current mantra is Blender (take a look at http://www.bensikora.com/tag/blender/).
Mind the Graph is an online platform of scientific illustrations with thousands illustrations available and you can request new ones. You can not draw illustrations, but you can use the illustrations available. (mindthegraph.com)
Here is a tutorial to create a graphical abstract using Mind the Graph: https://blog.mindthegraph.com/cell-signaling-pathways
But we were looking for a simple graphic designer tool, available for free preferably from any website. Photoshop is quite efficient tool and we're using it, but preparing illustrations and new 2D objects/models are not that easy. Anyway, thanks for your comment response Dr. Mohammad Reza Abdollahi .