I've just used pliers or small vise grips to spread open dropouts. Realize I was working on cheap bikes I didn't really care about, but the process is the same either way. Anything you can force/wedge in there will be able to open them up enough to fit a standard axle in. The dropout in your photo appears to be a stamped one which should be thinner and easier to bend than a thick forged DO like the one in Bill's photo.

Alternatively I suppose you could file the DO but that seems like a lot of work and I've never gone that route before.