To do this it's important that there is no "black spaces" between pictures otherwise the collage does not look like it's fitting together closely and it doesn't look that nice.
So to achieve this on the frame the only solution is to crop pictures that don't have the same ratio as the window in which they go for the collage. Sometimes by cropping we loose some part of the picture especially for portrait pictures and this may give results where the upper and lower part of the picture is not showing up (the "head is cut off").
This really depends on which window of the 5 picture collage the picture is located. Some windows the picture will fit better than others, but there is no way to avoid this if we want to really make a collage where there is no black spaces between pictures.
As an alternative to this, you can try our new collage 5 pix slide show that we've just launched which is called:
collage 5 pix (no crop)
this one does not crop pictures but then it's not a 'true' collage as there will be black spaces sometimes between picture, so the result is not as good as the other one probably. But if this is ok for you then you can just go for the this one.
At the end of the day, if you don't want any cropping then switch back to the normal mode where you have 1 picture at a time on the screen this will always preserver the picture ratio and won't do any cropping.
Enjoy your Pix-Star!