What can be inferred about the strength of the synapse from an electron microscope image? In the following EM image (1um scale), there are several synapses:

Image or HD Version

What information about the synaptic strengths can be extracted from this image?

Does vesicle count contain information?
What about the cross sectional "length" of the synaptic cleft?
If serial sections were used to reconstruct a 3D version of a synapse, would the surface area of the synapse reflect the strength of the synapse?
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.