On the Mac, you should be able to make ArtRage a window by clicking the icon just to the left of the X in the toolbar posted above. You can toggle it back to full screen with the same icon. The window is not resizable currently on the Mac due to the problem of the default OS X drag handle obscuring some of the interface when Windowed, so it is a fixed size.
However, if you are running the Full Edition of ArtRage and need to have a reference image open to work from, did you know you can load reference images directly into ArtRage and pin them on your canvas? They can be moved around, rotated, resized, zoomed into and out of and so on. This feature can be accessed by going to the Tools menu, and selecting "Load reference image", then choosing the image file you want to use. You can have a whole bunch of these open at the same time if you like.
Regarding the second question, in ArtRage 2.2 when you flip the image, there's not a way to keep it flipped. That's another thing we have noted down in our small ( hah! ) list of suggestions