There appear to be a great many photos surviving from this time showing women joining various militia organizations. Was this actually common and do we know what percentage of overall forces were women?
Was it limited to certain areas? Certain militias? Specific time frames (such as the beginning of the war with the mass mobilization of the unions)?
Or do we have so many photos of women fighters because it was unusual enough to document?
Female soldiers were mostly common in the Anarchist militas of the early stage of the war. This means they were mostly active during the inital coup d'etat. These female militias were pushed back and abolished once the Central government and the Soviet military advisers (re-)gained control.
And yes to your last sentence. These women militias weren't common and weren't 100% accepted. At the start of the war in the centers of the revolution they were somewhat accepted and admired but as soon as counter-revolutionary measures came into effect they were often ridiculed and subsequent abolished. And while later during the Second World War women were active in Partisan/ resistance circles, and had a role in the Red Army, they never came close to building all-female corps/ fighting groups.
You can read up on Mujeres Libres, as they were the main anarchi-feminist group with a pretty large following.
Further readings:
Beevor, The Spanish Civil War
Lines, Female Combatants in the Spanish Civil War: Milicianas on the Front Lines and in the Rearguard
http://vc.bridgew.edu/cgi/viewcontent.cgi?article=1265&context=jiws