If point a and point b have known coordinates, like gps, then most maps like Google or Bing can tell you distance. If your looking for exact results, first you would need to figure the exact locations of points a and b on earth. Several softwares such as pix4d, or agisoft can do 3d renderings from the images (and the gps data they contain) and combined with ground control points, (ones you would include in your shots that have known exact gps locations), to produce a model of your location. This method still relies on interpolation of locations for points a and b, however it will give you a distance within a set certainty or percent error. I take my 3d reconstructions and draw them in arcGIS for additional calculations such as stratigraphic thickness or even volume. . . It can be done, just depends on how certain you need to be of the exact distance. . . And how much work you want to put into obtaining your measurement. The use of two or three additional software to do this can be a bit daunting for what Google Earth can tell you with a couple clicks but more approximated. I hope this answers your question.
