Custom thumbnail size help

Hi.
I've just been looking at Gallery 2.3 and it's almost perfect for my needs, except for one thing which I'm hoping is a simple theme / config fix.

I've already searched for an answer here, but haven't found anything.

Basically, I'm using the Slider theme, and want the portrait and landscape thumbnails to have the same height.
For example, a landscape thumbnail will be 160x120, whereas a portrait thumbnail will be 90x120 - both have a vertical height of 120.

Can someone point me in the right direction on how to achieve this? I'm handy with php, so I don't mind getting my fingers wet with the code - even to just hard-code the dimensions in somewhere.

Thanks in advance,
- Nigel