Hello BriniaSona.

I'm no expert, but most of my world maps are equirectangular or similar (none currently uploaded to the Guild unfortunately), and this does look very much like an equirectangular map to me - mostly because by my crude measurements (done with the edge of an envelope against the screen) its more or less twice as long as it is tall - equirectangular.

Mercator tends to grossly extend the polar regions and never reaches the poles, which lie at infinity in that type of projection.

However, that doesn't really help you with the conversion.

I would suggest you follow Falconius's advice - he being a more knowledgeable chap than me