is it possible to convert a video to set of images *while keeping them in high quality*?

I am currently using ffmpeg for this but the quality of the images seem to be quite bad

You mean programatically? After Effects / Premiere can do this.
However, ffmpeg should be fine.

What are your settings? You can set the output quality with

-qscale:v 2 The range you can set is 2-31 I think while 31 is the worst quality.

There are also a lot of plugins for ffmpeg so you can output tiffs, of jpegls for lossless jpgs