You need to make a jpg of the map then after you have loaded a track in gpsar (file ...open then click on a track you have downloaded to your computer from your gps in gpx format by using G7toWin). Then when you have the track displayed on a blank background in gpsar hit file again, go down to "create new map". A popup will appear. Find where you saved the relevant jpg, hit open and the jpg (map) will fill the background under the track on the screen. Now you need to be able to recognise at least two places on the track to known places on the jpg map. The next step is trickier than Yann would have you believe but while holding Shift + Ctrl down together, with the left click of the mouse drag a point on the map to the point on the gps track that you know represents it. Do this for another point some distance away and the map should be calibrated. Then go back to file and go down to "save map", hit that and another popup will appear. Put in whatever name you want to save it under whereever you want to save it but make sure you put a ".maps" tag on the end.
Ok, now you have the map (jpg) and the calibration file (.maps) hopefully in the same folder. In future when you load into gpsar another gpx file of that area you want to view. After gpsar loads the track on the screen hit load map, go to where you saved the map and .maps file , hit open and your map should appear.