For pretty much for every OST Composing jam my process has been the same:
I put the prompt photo on my second monitor with the text prompt, then i load up a piano sound in my DAW (usually an acoustic piano sound) and start improvising stuff while "passionately" staring at the prompts. Usually i record 15 minutes or more of material, then depending on what time it is, i go make a cup of coffee or tea and after that i listen to the recorded stuff again and if something sounds good to me, i start working forward from that sketch, but i usually end up trashing the first idea the next day and going with another idea from the same improv recording, or from some other improv i recorded later on the day.
When going forward from the piano sketch, i don't really have a defined "ensemble" or "palette" of instruments that i use everytime. I usually take these jams as a opportunity/motivation to practice/try different genres/styles, since normally without a goal or deadline i won't actually get anything done.
I went with mostly orchestral instruments for this jam, since i've had the EastWest Hollywood Orchestra library for some years now and those samples are very dry without much reverb, so i've had a lot of issues understanding how to apply reverb to those samples without overdoing (which is what usually happens to me with them). I also recorded some vocal stuff with the intention of learning how to blend dry recordings with other stuff, so that they sound like they are in the same space. The vocal recordings turned out a bit so-so, but i feel i'm beginning to understand applying reverb to dry sounds and making things sound in the same space with this jam.
After i chose to go with the orchestral sound route for this one, i started thinking of the arrangement.
I had done a Joe Hisaishi/Anime inspired track for the last jam that sadly ended up being just the initial piano sketch, since i was too lazy/slow on doing an actual arrangement of it, i had decided that for this jam i would go a different route in the harmony/style department even though the image has a lot of Asia influence in it.
So from that, the initial theme sketch turned into the sort of generic starting the voyage sounding track "Daydream", which is probably the polar opposite of the picture theme to most people, but oh well.
After i felt reasonably happy happy with "Daydream", i started working on a second track with the intention of having it be a sort of action track, since the main inspiration i had for the game setting was some sort of kingdom hearts style game. where the player enters different dream worlds during their daydreams.
I started the process again by improvising on a piano sound, this time using "Daydream" as sort of style guide during the sketching process. Then i did the same old long ass improv recording, made something to drink, came back, chose an idea from what i recorded earlier and started arranging the track.
For the second track i was a bit out of ideas for a while, until i remembered recommending the trial of the Musio subscription service in the jam discord. So i went ahead and got the Musio plugin set up, found while browsing that they had a collaboration library with the cello metal band Apocalyptica, downloaded those and after playing with them for a bit, got the inspiration/idea for what turned out as the second track "Twisted Dream".
After that it was just mixing and mastering the tracks, trying to not squash the dynamics too much and submitting the project
This ended up as a big wall of text, but hopefully someone gets some use out of my ramblings.