Sure there is an easier way of doing what you need, and also any other batch change in article...
I've been doing it for ages...
But for that you have to use editor more suitable for programmers than writers.
That editor is RocketTheme RokPad.
Once you install it and set it as default editor (you can also use Editor Switcher if you want to switch editors within article editing screen), among many options, in toolbar there is "Find and Replace" command (see below)

Now comes important and harder part. I cannot tell you exactely what to enter in Find
and Replace
boxes, that you will have to figure by yourself.
I can give you just an example. Find style="
and Replace with style="margin:20px;
.
There are some drawbacks though:
- What if you have other
style="
parts which don't belong to <img>
tag?
- What if
<img>
tag has no style=""
part at all?
- etc...
Exact strings depend of your specific article content, but you got an idea now, right?
EDIT:
To be perfectly honest, before I have discovered this gem of RT RokPad three years ago, I've been using another way of achieving same result:
- I would [Toggle Editor] (JCE of course) to source code
- Ctrl+A then Ctrl+C
- Switch to my favorite Windows editor (IDM UltraEdit of course)
- Ctrl+V and edit it to my liking, by using all powerfull UE commands
- When finished editing I would copy it and paste back to JCE
Same result, just little bit longer, and not as fancy-schmancy as with RokPad. :-)