RESTful API urls best practice GET vs POST

I am trying to create shopping app for android for that I am building REST API on my store site.

I wanna optimize and bring user better experience.

So my problem is that images of products, categories and other parts of store are resized according to web browser, so have relatively large size to mobile device.

So I decided to optimize this passing current width and height in request.

So I wonder what is the best practice and look of url to do this.

Now I have following url

http://ift.tt/1DyLgvw

So first time app is installed image settings is saved somewhere on the device and than used to request product using web site API.

I have thought about something like this

   http://ift.tt/1bfYHHC 

But it seems weird what width and height does mean in this case ?

Also I thought about passing parameters in POST request, but POST request is used for storing something on the server.

So please suggest how url should look like in my case , what is the best practice or doing so.

Thanks in advance.