Consider the glosses (I lean heavily on wikipedia, here). The accusation against Charles is that he was a tyrant. How, might I ask, is that countered by having him with his foot upon the world, and the Latin tag 'Mundi Calco — I tread on the world.' Isn't that exactly what tyrants do? What does Lewis and Short say about calco? 'to tread something, or upon something ... to tread down, to oppress, to trample upon ... to scorn, contemn, spurn, despise, abuse.' Is that really the semantic field of a wise and benevolent ruler? (abuse? oppress?).
Those books on the table: IN VERBO TVO SPES MEA — "In Thy Word is My Hope" and Christi Tracto — "I entreat Christ" or "By the word of Christ". Fine, except that Charles isn't looking at them (he's looking up). 'Tracto' might mean 'I entreat Christ'; or it might mean 'I ponder Christ'; but L&S remind us that its primary meaning is 'to draw violently, to drag, tug, haul' and goes on to expand the possible uses of the word re: violence ('to be torn, rent, lacerated') or 'to strike'. The Roman soldiers at the crucifiction might be said to have traxerunt Christ, surely? And in verbo tuo spes mea is, at the least, ambiguous depending upon to whom the tuo and the mea refer?
That rock in the ocean is immota, triumphans, which of course suggests Charles beset by a troublous populace yet remaining unmoved and triumphant. But does a monarch accused of imperial tyranny really want the associations of the Roman triumph? Is it fitting that he has (perhaps Pharaonic) palm trees in his garden? With bells hanging from them (who would not think of 1 Corinthians 13's sounding brass?).
Most of all I'm intrigued by the line of the beam of light. Ostensible the light from the king (from the back of his head) shines upon the darkness, whilst he himself had his eyes fixed upon the heavenly crown that awaits him. But if we look again: doesn't it rather look as though Charles head (let's not forget, King Charles's Head) prisms the light of heaven into a spectrum of ... uh, cloudy darkness? (What a shame Newton's Optics postdates the Eikon by decades, or I could really go to town on this).