When choosing images for sentence flashcards, your image choices will depend upon the sort of word that you're learning. Let's say your sentence is "Those dogs ate the hamburgers."
If you're learning something concrete, like "ate," then you can get images of:
1. The word (people/dogs/whatever eating things)
2. The word, but aiming for images that fit this context a bit more directly (dog eating)
3. The story (literally dogs eating hamburgers)
All of these choices are good. That said, if I know I'm going to learn a few different words from this sentence, then I'll go for #1 or #2, so that my associations are more specific to this particular word.
If you're learning an abstract word, like "the," then your choices are much more limited:
1. Some part of the story that's closely tied to the abstract word (hamburgers, eating)
2. The whole story (dogs eating hamburgers)
Either option works just fine. If I'm learning a few different words in this sentence, I'll tend towards #1.
Some words will fall into a grey zone, like "Those." They're pretty abstract, but they may bring some visual associations to mind (for me, when I think "those," I think of pointing towards things). So you could do:
1. Visual associations that fit the context (pointing, or someone pointing at dogs)
2. Some part of the story that's closely tied to the abstract word (dogs)
3. The whole story (dogs eating hamburgers)
When an association comes to mind, then be sure to use it (#1). Otherwise, treat it like any other abstract word.