Image alt tags are handled by core WordPress functionality, and are not affected by the theme.
Alt tags are designed to be descriptive text used by screen readers and search engines, and the alt tag should describe the image in a single sentence.
It's a good idea to use your keywords in the alt tag if the image is related to your keyword, but do not use the keyword if the image is unrelated.
Here are Google's guidelines:
Do not keyword stuff alt tags, and do not fill them with excessive content.
Image names should also be descriptive, and related to the content. A huge mistake bloggers make when uploading images is that they leave the files as the camera named them - DSC, IMG, etc - this is a lost opportunity. Image names should adhere closely to alt tags - descriptive, accurate, and keyword-driven. Examples for a spaghetti bolognese recipe might include:
- spaghetti-bolognese-turkey.jpg (if using turkey, for example)
Here's a quote from the Google image guide above:
- Create good URL structure for your images: Google uses the URL path as well as the file name to help it understand your images. Consider organizing your image content so that URLs are constructed logically.
In the past, alt tags were used by food bloggers for managing Pinterest pins, but this is no longer the best practice. Here's a write-up from Tasty Pins: https://www.wptasty.com/blog/how-to-craft-alt-text-for-images