What can be inferred about the strength of the synapse from an electron microscope image?
Vesicle count and spine morphology are two of the better-known predictors of synaptic strength.  But they are not the whole story.

Glia, like astrocytes can also modulate synaptic strength by releasing their own neurotransmitter or co-agonist, but you need more than just images to detect that process.  There is also the matter of receptor distribution in the postsynaptic cell inside the synapse.  Even if you make more vesicles, it won't matter if there's not enough receptors.  It also depends on how much agonist, co-agonist, and antagonist are present, which could be the result of any particular molecular signaling cascade.  There are also modulators which can increase or decrease the effectiveness of a particular receptor.