Creating a formatter for multi-value field to be used with insert

Here's my situation: I have an image field that allows for unlimited values. It's part of my post content type - the first image added is used as a thumbnail for the post, while others are only displayed if the user chooses to insert them into the post body using the Insert module.

I need to define a formatter to be used by Insert when inserting images, e.g. a certain type of thumbnail with a link to the original file. I know how to make such a formatter for a single-value field and I also know how to make one for a multi-valued field that loops through all of the values and inserts them, but this is somewhere in between. The field itself is multi-value, but the Insert module is only inserting one of the values (only one image) and needs a formatter for that.

If it is of any help, I used a formatter for a single-value image field a couple of months ago and it was working. Don't know whether it was your update or Insert update that broke it for me.

Hope you can point me in the right direction, I'll be asking the Insert people too in case it's their fault ;).