The kind of animation done in Spine is quite different than Flash.
Flash is frame based and drawing tool, each frame can have a different drawing than the previous.
Spine is skeleton based.
Just like skinned 3D, you can't use a completely different model each keyframe.
It works transforming the setup model.

You can get frames to play in Spine animating attachments.
That dragon in the example files show it.

Did you use your eyes to see the part that I wrote that it can be done animating attachments?
If it's not what you are looking for, good luck waiting for Flash features in Spine.
I'll enjoy them too.

Simmer down guys!

Spine would need to complete with Photoshop, which would be quite a feat. It's probably better for Spine to focus on what it does well. Note Spine watches the images and automatically reloads them if they change.

Point 1:
Spine is primarily for skeletal 2D animation for games. Or skeletal 2D animation for other computery things. It's skeletal. It's designed this way to address certain needs that game animation and the game development process has, and the underlying system Spine has now is built to answer those needs.

You don't go to a Chinese restaurant and ask them to serve a mean spaghetti just because you love the Fusion restaurant down the street.

Point 2:
Purely for the purposes of having guides for poses and timing, having simple drawing tools (a sketch pencil + eraser + some simple selection and transform tools) within Spine Editor itself would be nice. It's much lower friction, more natural, faster and nicer for a trained animator to draw poses out instead of rotating bones one by one to fiddle around and guess at what a good pose is.

... which is why I requested this feature many, many, many, many months ago: viewtopic.php?f=11&t=1806&p=9023

But @kanevsky, this isn't what you're looking for, I guess. You want to draw, color, paint and time your animations in one program. In that case, you want Flash. or ToonBoom, or TVPaint or something.
Spine doesn't have the slightest inking of becoming anything like those programs with all those brushes and panels and layers.

Point 3:
Spine is 2D and not 3D. That means it can't rely on the things that make 3D look good and dynamic. Because of that, it can't fall back and ride on just standard 3D features 'cause those are arguably enough if you can rotate the model or the camera, but it would mean too many simple-but-good-looking traditionally-animated things become too difficult or impractical to do.

Luckily, Spine allows attachment swapping, and some FFD and skinning/weights functionality already exist.
But it's still missing some features that facilitate things like some animated, full rig or bone-and-image swaps; things other visually impressive, skeletal-animated games were able to do more than 5 years ago (Muramasa: The Demon Blade and Odin Sphere), and continue to do so today (Rayman Legends)

Some people using Spine have managed to cobble together similar looks but I can only imagine either the complexity of their setups or the great pains they had to undergo, maybe wresting with the interface at some point, just to get it to work correctly as a system of animations and look good (Onikira), not just have single looping animations that look good but do nothing else. Spine can do it today, but at an arguably medium-high visual fidelity, there's room for some nice supporting features in there to make it easy.

But these examples show that skeletal animation rivals and surpasses traditional animation when it incorporates aspects of traditional animation well.

Now for an analogy..
uh..
Good Chinese restaurants... serve broccoli dishes as their specialty. And also use carrots.
There. Nailed it.

Point 4:
Let's be real, Nate. Moo has the last say on these things.

Good points, you have.

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I expect the OP wants drawing tools in Spine for making new images to swap. It's still took much effort to compete with dedicated image tools. Simple sketching for animation purposes may make it in (ie grease pen).

What are the fancy things Muramasa and Rayman does?