GIMP Version: 2.8.14
Operating System: Mac OS
OS Version: ElCapitan
GIMP Experience: New User

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I am going on a trip to the Yucatan in May and want to take good pictures inside some of the Cenotes (sinkoles that expose groundwater underneath). Some of these cenotes are underground with not much light (Beams of light from the sun come through from the holes at the top of the cave, and some stationary artificial lighting is provided from the people who work at these caves as well).

So my plan is to use my Rokinon 12mm f2.0 with my Sony A6000 on a tripod and set it to max aperture and long exposure, with maybe ISO of about 400 to 800. Do those settings sound about right to get some good low-lit cave pictures? Would there be any reason to lower the aperture? Keep in mind, I do not plan on going any further and setting up external light sources, etc. The cave should be lit enough to be able to grab a decent picture with long exposure.

Anyway, the more important question or concern I have comes into play when I want to get some action shots. Long exposures and people moving don't mesh, so I have to take a short exposure frame with flash from the same angle with the person, and then insert that person into the long exposure photo. For example, freeze the person when jumping into the water (in the air mid-jump). I think the only way to do that is with a computer program using layers or masks, correct? I don’t want to purchase anything, and I think this can be accomplished with GIMP on my Macbook pro.

Last night, I tried masking myself out of a picture (or whatever the term is), but it was very hard to do in terms of being precise. I tried to fiddle around with the lasso and the different detections (color/shape/contrast), to no avail. Maybe part of that was because I was on a small laptop without using a mouse. But I think most of it was because I am an extreme beginner and this is my first time playing around with photo editing tools.

Can someone please point me to some detailed Gimp tutorial videos either showing how to accomplish exactly what I want from start to finish (Add the frozen person from the short exposure frame to the long exposure frame), or if you don’t know of a video that explains all of that, then maybe some tutorials on something similar, like how to precisely mask out people.

UPDATE: I have included a picture of what I am trying to accomplish. The cave in question is pretty dark, so I am almost positive he used the method of layer masking to insert his self into a long exposure shot of that cave, from a short exposure flash induced shot with the same angle