I enclose another scanned image. This shows flimsies being unloaded at the tankodrome amidst a number of sponsonless Mk Is. The date is later than April 1917 as in the far background sits a Mk IV female (I enclose a blowup). The Mk Is are not passive hulks as exhaust fumes show motors running. They do not appear to be in the midst of conversion to supply (and in any case the number on the tank nearest the camera indicates it was female). What were these tanks being used for?
The two numbered tanks in that photo are 564 and 528, both female, but I would say that they are not running. The exhaust fumes originate from a point beyond the Mk IV female in the background. I've marked this out. Notice the way the fumes dissipate and expand the further you get from the source. This would not happen anywhere near as much with a tank running close to the camera. Another point is that if the exhaust was from 564, it would obscure the trestles on the tanks behind 564.
There is also what looks to be a male tank hiding just behind the back of the truck.
528 is a veteran of Flers-Courcelette (D23) which lost a track while moving up in support of the 15th Div. near Martinpuich. It was reported as repaired by Sept. 17,1916. The Mk.I's were said to have been in generally poor mechanical condition by the spring of 1917, and D Bn. re-equipped with Mk.IV's in July 1917. Driver training was often done with sponsons removed, so if these tanks were still runners, perhaps that was their use. No info on 564 yet, other than it was not on the roster of C and D Co. vehicles on Sept. 15, 1916.