Doesn't look like it, using the IMAGE formula directly. A few options I would suggest.
- as part of your normal business process, re-scale the images upon upload. Obviously depending on the process and/or personnel involved, or if they are self-service uploads, this may not be feasible.
- do a nightly / hourly / whatever rescale of all new images. This would be a pretty easy task via an API integration and the excellent ImageMagick library.
- if changing your upload process to introduce a scaling step, or a nightly rescale, is undesirable, you could introduce a middle-man server to do the scaling on the fly. So a URL like
'http://posse.force.com/photos/servlet/servlet.FileDownload?file='+LEFT(SYSTEM_Photo_Id__c,15) becomes something more like
For option 3, there is a discussion here with some interesting options, including the little tidbit that Google Docs supports dynamic image resizing which I was not aware of. Image Resizer also seems like a pretty good server option, and it's free.
EDIT 2: just found this thread which has what look to be pretty viable middle-man options that are already around and look pretty easy to use (and mostly free!), if you don't mind a third party processing your images.