When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
One of the easier ways is to open the picture in Microsoft Paint, which I believe is included for free on any computer running Windows. I assume Apple has a similar editor.
Open the pic in Paint. Draw the arrows, add text or whatever, save the pic.... simple as that.
Also there are lots of free picture editors that can be downloaded. On of the easiest to use is Paint.net, which is similar to MS Paint but with a lot more tools and options.