This has always been kind of a weird thing in Photoshop. The Save settings try to be “sticky,” based on previous saves, and/or characteristics of the file when opened. But, there are a lot of little factors that can come into play, like the presence of layers, transparency, color modes, bit depth, etc.
Not every format supports every image attribute. So for instance, if you open a GIF, make a selection, change a color, and hit Ctrl+S, Photoshop will “save over” the original in the same .gif format. But, open that same GIF and change its color space from Indexed to RGB, then add a layer. Now hit Ctrl+S, and Photoshop’s Save As dialog will come up offering a format that supports layers and RGB…GIF doesn’t…most likely TIFF or PSD.
I don’t think there’s a way to force a given format as default for saves, but once you understand the behavior I touched on above, it starts to make sense, and you’ll learn when and why to expect it.
John M:
All of the vim with none of that annoying vigor.