I've created a few rocks using blender, what do you think?

Is there shadow issues with your rocks? on the left there? Also the texture looks a bit blurry?

Good start with just blender, Try shifting the lighting To one side. Usually Mid day is the worst TOD time to shocase assets, it makes it hard to see the details and Silloutte of the object making it seem kind of blobby with no detail...

As far as the texture goes, try tiling it. See if you can get something more detailed.. Whats the resolution of your texture? and do you have a normal map, and detail map applied?

I have to admire your spunk! Rock modeling is very tough to get right, and without Zbrush or mudbox it will be a great challenge. But your off to a good start.

My texture map is 2115 x 1494 px.
I have set the time of the day to an other time:

Oh yikes!

If you can, keep the Textures Square.. as in 2048x2048, or 1024x1024 pixels.. This is to save precious memory, and images that are Binary load a good deal faster.
The lighting now complements your model better.

I have a suggestion:

Download Xnormal: http://www.xnormal.net/Downloads.aspx

And set your large rock model as the HP and the LP.. and try baking the normals for it, and adding them to the Detail map for your rocks.

My skype is themodman101, if you need help im almost always on

Thanks, I will see what I can improve...
Here are some of my rock models:
http://www.mediafire.com/?4pr9uhrprgazdne

What do you think?

I now have 9 rocks, any suggestions?

Yes, make more 

Some cliff-size models are usable too. Having a human model in the scenes would help visualize the scale of the rocks.

Here is the image again, with the human model (attachment).
Do you have a picture of a cliff-size rock?

Unfortunately i haven't pictures about cliffs, but they are easy to find by searching the net.

Thanks for the new image 

Do you mean something like this:

Yep, but smaller ones too like in Crysis and Crysis 2.

I've made this cliff very fast, but do you mean something like this one?
Sorry for my bad English...

Yes, cliff looks quite good overall and in size...

...and your english is fine too 

Thanks, I will make some more cliffs and rocks and then I will release them.