Where do I put image style definition in a module? I'm writing a custom node formatter that needs to use a certain image style. Since I don't want to rely on that image style being defined by the admin, and I want to be sure the style is not deleted, I am going to provide the style definition in code.

Should MYMODULE_image_default_styles go into the .install or .module file?
I think its better to use features and export it, then in your .info file add dependency to it.

other solution is what you say, put it inside MYMODULE_image_default_styles in your .module and re-install your module to make it locked in admin