how can i see scale and grid on exported images?

You can add them with any image editing software. If you want a more customized map you can try out TileMill.

You can ahve a look at coordinate-lines-in-josm ... there is a solution to get grid lines in any *.osm file via an editor.

Or have a look at Maposmatic

You can try using qgis that have a proper printing tool.

You have to calculate it yourself and add it with a image editing software (GIMP or Inkscape or the like).

resolution = 156543.034 meters/pixel * cos(latitude) / (2 ^ zoomlevel)

The resolution means how many meters per pixel you get.

scale = 1 : (dpi * 39.37 in/m * resolution)

The scale means, how many cm in reality is 1 cm on the paper (or on the screen).

So if you have a screen with 96 dpi, you get that one pixel is 1.1943 meters. And you get a scale of 1 : 4 231 which means that 1 cm on your screen is 42.3 m in reality.

If you have a printer which prints 300 dpi … (now do the calculations yourself).

There is a wiki page about scaling.