Why do you need alternative texts at all?
All figures that show relevant content - for example logos, symbols, photographs, diagrams or formulas - need an alternative text. This requirement is obvious: when you can not see the figure, it should "speak" to you - and preferably something which is meaningful.
Which rules should I consider?
I can see what you can not see
The basic rule about creating alternative texts is: what would you tell another person on the phone to communicate the content of the figure.
As its name says: an alternative text is thought as an alternative to the figure and not an addition. It should describe the quintessence of the figure and need not to be a detailed description.
Two is not better than one
Please avoid to use an available caption as an alternative text - this is saying the same thing twice over. A caption is perceptible for everyone and adds something to the figure - an alternative text in contrast replaces the figure. Double naming is at least annoying or even confusing.
Omit introducing words like "The picture shows ..." and come to business.
If a figure contains text, than the alternative text should pick up exact this text. In case of the logo of a company this could be the company name and corresponding slogan.
Summarizing is allowed
It is often difficult to write alternative texts for figures like bar charts, pie charts, coordinate systems or curve charts without writing half a novel. Instead try to get the trend of a chart to the point.
By the way: when the data of a diagram is already mentioned in the text then it is enough to write "SBar chart about topic xyz" as alternative text.
Brevity is the soul of wit
As rule of thumb: not longer than a text message (that means only 160 characters!).
Good to know: on average a braille-line has about 80 characters.
But in case of data diagrams you can break this rule sometimes.
"Pie chart with the results of the opinion poll from august: 45% Yes | 55% No"
"Bar chart: Electricity crop in spring, summer, autumn and winter; values from the mornings, noons and evenings respectively. The values in the noons are always the highest; peak value is in summer."
"Screenshot: Word dialog box "Modify Style" with the settings of "Heading 1"."