If I were to close my eyes, how would you describe to me what your artwork looks like?
In a nutshell, that is the role of alt text - or alternative text - and it's important for 2x main reasons:
1) Accessibility > allowing people who can't view web pages, due to impaired vision or poor internet connections, to understand what is appearing on their screen
2) Search engine discovery > allowing search algorithms (e.g. Google search and even Bluethumb site search) to scan and understand the subject matter of an image
The goal here is to be as succinct as possible. Avoid 'stuffing' irrelevant keywords into your alt text. There is no benefit.
Likewise it is not essential to include 'oil painting', 'sculpture' etc. as we capture this elsewhere in the upload process.
Here's an example of image alt text. Aim to 'say what you see' in 6 words max.
Good: Pink flowers in vase
Best: Pink roses in Art Deco vase
Adding descriptive alt text whenever you upload an artwork is a small extra step, that brings a large range of 'behind the scenes' benefits to your artwork.