I have not used it but I know ImageJ is a useful tool if you are looking for particle size from SEM and/or TEM images. But I do not think that can be used for particle size distribution analysis.
Every things depend on your samples. if you have a powder you could use SEM images , i also recommend, use FESEM for better resolution and image quality, if you couldnt identify the shape and size of your particles,then you have to use TEM images. TEM is recommended for nano scale particles. here you may note that the shape of your particle such as nano spherical or nano fiber or nano tube shapes. if nano particles were dispersed in your film, may you have to use EDXA for distribution/dispersion investigatons.