Thanks for your help, I have tried the 'crop thumbnails to exact dimensions' setting, but it only seems to control whether image is forced into the thumbnail size you've selected - it doesn't seem to actually crop, just resize.
So, if I insert a portrait photo from Picasa and I use the 'crop thumbnails to exact dimensions' option, then the actual resulting thumbnail is 195x291 pixels (right clicking on it and selecting "View Image"), but its squeezed into my 195x150 box.
If I insert the same photo and do not use the 'crop thumbnails to exact dimensions' option, then the resulting imaage is still 195x291 pixels, but in my post, the thumbnail box is resized to 195x291, so its not squeezed, but its now not the shape I'd like.
Ideally, I'd like the photo to be resized as close as possible to the thumbnail size selection while retaining ratio (eg. size to 195x291 for thumbnail selection of 195x150). From there, I'd like to be able to select an area to crop (eg. "top", "middle" or "bottom) to generate a thumbnail picture to meet the thumbnail size selection (eg. 195x150).
Sorry, I appreciate that I'm probably trying to do something that's quite particular. If its not possible, its not a big problem - I'm sure I can maybe try and find some way around it, like manually uploading replacement thumbnails via FTP or something.