Kotti provides on-the-fly image scaling by utilizing plone.scale.
Images can be referenced by this URL schema: /path/to/image_content_object/image[/<image_scale>]/download] where <image_scale> is a predefined image scale (see below).
If the last URL path segment is download, the image will be served with Content-disposition: attachment otherwise it will be served with Content-disposition: inline.
You may define image scale sizes in your .ini file by setting values for kotti.image_scales.<scale_name> to values of the form <max_width>x<max_height> (e.g. kotti.image_scales.thumb = 160x120 with the resulting scale name thumb).
span1 (60x120) to span12 (1160x2320) are always defined (with values corresponding to the Twitter Bootstrap default grid sizes), but their values can be overwritten by setting kotti.image_scales.span<N> to different values in your .ini file.