Here is an absolutely outstanding resource, that will tell you everything you need to know, and then some, about image description. Added bonus, really cool pics of a guide dog teaching you how to do it correctly. https://www.ssa.gov/accessibility/files/SSA_Alternative_Text_Guide.pdf

I feel like there are two different approaches to captioning instagram. I do a lot of IDs on tumblr, and I will do them completely differently if it's like, a reaction gif I'm describing, or if it's like a piece of fan art. For example:
\[ID: Will Ferrell, holding a drink and looking shocked, with text reading "That escalated quickly."\]

and you can totally do that, just being careful to make the correct assumptions about what your audience knows- if you followers are mostly 12 or Amish they might not know who Will Ferrell is. But most of your followers should know who you are, so you can probably put like "Eli is smiling and petting a horse" rather than like "Eli, a tall, purple skinned alien with tentacle hair, is smiling a petting a horse"

However, you can also treat your posts like art, and assume people WANT to know the details, and kind of 'set the scene' a little bit better. Example:

\[ID: Wide shot of Eli, a tall, purple skinned alien with tentacle hair pulled back in a messy bun, smiling excitedly and facing the camera while petting a tall, white horse. Eli is wearing a long sleeved white shirt, a brown vest, black trousers, and brown boots that come up to their knees. Eli and the horse are in a field, surrounded by grass covered in fallen leaves, trees looking out of focus in the background in bright yellows, reds and oranges.\]

It sort of establishes that this is an artsy, fall themed photo and gives the vibe of it, not just the bare bones details. But it all comes down to what you want to say and what message you want to get across.

And one more thing! I know this is VI forum, but other people do use the image descriptions! As an autistic person, I love when someone says what facial expression a person in a picture is making because I cannot read faces to save my life. I am also colorblind, and I love IDs for that as well (when I do them myself, more than once I've had someone be like "Thanks for the ID but that object was brown, not purple" lol.)

Lastly! Thank you for doing this. Accessibility is so important, and by doing Image Descriptions, you're not only supporting blind/VI/autistic/colorblind/anyone else who likes reading more than pictures, but you're also making your sighted/abled audience aware of this, and maybe they will think of accessibility for all in their own lives too! You rock!

Describe the main topic with a bit of the important other details thrown in. No need to describe everything; something like your example is good with a few more details like expression or whatever it is that made you take the pic in the first place.