Use multiple choices of CSS class in Insert If using Insert module, in the field editing, we can put Additional CSS classes to the image being inserted.

I need the images float right. So I type floatrimg in the field editing, and .floatrimg {float:right;margin:0 0 10px 10px;} in CSS. It works fine.

But if I need to have 2 images in one post: one image float right, another one float left, I need 2 image fields.

I hope I can have one image field, where multiple choices of CSS class can be selected when submitting. So that with one image field, I can insert image float right and float left.
I would suggest you to create two presets. Each preset can then be styled and positioned with CSS and the .imagepresetname class. You just have to activate the presets you like on the field in the "Insert" settings.