I like your landmasses! When it comes to mountain ranges, there's one simple rule: mountains either run along the coast, or they steer clear of the coast altogether. Mountains never ever are perpendicular to a coastline. Apart from that, you can do pretty much whatever you want!
Seems like you have some nice longitudinal features that could benefit from a hefty mountain spine. From the top of my head I'd say...
- A mountain range along the western coast of the southeastern island;
- another across the southern peninsula of the mainland;
- another along the isthmus that links the peninsula to the rest of the mainland (maybe linked to the previous one);
- and a series of ranges across the northern mainland, wherever you'd want them. One hugging the western coast wouldn't be bad, for example.
All in all, this map gives you the opportunity to go wild! So go ahead and start posting some sketches