As I use mementeo among others to track my daily ideas, sometimes I use the "subscribe" field to scribble something down ad hoc, sometimes my note is longer and contains pictures in a picture field. Both, the picture and the handwriting, are displayed as a picture but I have to choose just one for the preview. So some notes will either way not have a preview depending on what I have noted, although they do have a picture.
It would be great if there was the option for the library to choose the source for the picture, maybe hierarchically (if there ist no handwriting, choose picture) or to be able to set it for each note individually (which would of course slow down the work flow a little).