There is not a way to manually choose which image in a note is displayed in the snippet; Evernote will choose the image with the largest smallest dimension.
For example, if you have 3 images with the following dimensions:
75x100
100x400
200x300
The "smallest" dimensions are 75, 100, and 200, respectively. We're going to use the image with the largest of those "smallest" dimensions (i.e., 200).
The only way to "choose" which image is shown in the snippet is to manipulate the dimensions of the photo using a third party application.