When screen reader reads the page to visually impaired person, she will hear something like:
Image. Described as "enter image description here".
Details may vary, some readers can (could) be configured to also tell the size and is it color or bw images, but that's merely a bonus and not always in use as it wastes time (hearing is slower than seeing).
So yes, by all means you should add descriptions. If you don't have patience to describe images properly, you should write1 something like
Illustration of the issue as described in text above
so they know they aren't missing any crucial information.
Good practices for visually impaired people also include:
- Describe problem well enough your Q or A makes sense even without the image
- Don't rely on the color only
- Don't rely on minute details only
- Make symbols really distinct (° and * are bad pair, O and + are good pair)
That way people that have visual issues but are not completely blind have a much higher chance to use site as a fully sighted person would.
1 Some manuals advocate leaving description blank if image is totally not needed, but in my experience large images without description raised some questions from the test users from the Polish Association of the Blind.
Source: Years ago I was one of the people responsible for making websites owned by the city hall of my capital city accessible.