[Logician]

Tim's solution is wise and it forces the image to resize with HTML. But it is inevitable that all images will be forced to be in that size if you use HTML.

To make some (largers) resize and others (smalls) stay as it is you need to handle the job in PHP, not in HTML and to handle it your code needs to know what is the image size so that it can take actions accordingly.

Unfortunately I cant see an easy way to manage your goal in PHP. AFAIK the image size of the attachment is not saved in the attachment table so its not possible to know the dimensions of the image by checking your db. There is a special function "getimagesize" in PHP which would be a remedy for your problem but its not a default PHP function but a part of an additional image library. In other words so as to use this function (and to get the image size) your PHP should be equipped with image library by installing it. It's a server side library so your host must be installing it and for the same reason even if the solution is provided, it will not work for all users since many hosts wouldnt include this library in their PHP configuration.

As for multi-attachments hack: It's a long waited hack but not released here. Neo says he is working on it for a long time but if you make a list of hacks Neo says he is working on, you can really doubt if he can finish any of this very long list soon. I also know there is commercial version of this hack coded by someone and sold commercially. But I dont know his name..