3:4 seems to work for mechs. I use 309x412 (309x418/618x836 is the default size embedded in most official RS products).
Aero units definitely get warped using a 4:3 image. I haven't tested extensively, but it seems a 7:4 (350x200) appears to display properly.
Vehicles likewise don't like 4:3. 3:2 (309x206) is much more true to the source image. I had to tinker with that a bit because official RS vehicle images are often in the neighborhood of 309x247 (which gets distorted in MML)