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HTML Standard:

The most general rule to consider when writing alternative text is the following: the intent is that replacing every image with the text of its alt attribute not change the meaning of the page.

So, in general, alternative text can be written by considering what one would have written had one not been able to include the image.

A corollary to this is that the `alt` attribute’s value should never contain text that could be considered the image’s *caption*, *title*, or *legend*. It is supposed to contain replacement text that could be used by users instead of the image; it is not meant to supplement the image. The title attribute can be used for supplemental information.

Another corollary is that the `alt` attribute’s value should not repeat information that is already provided in the prose next to the image.

Tumblr’s theme documentation:

{PhotoAlt}: The HTML-safe version of the caption (if one exists) of this post. [Example usage:]

<img src="{PhotoURL-500}" alt="{PhotoAlt}"/>

{block:Caption}
    <div class="caption">{Caption}</div>
{/block:Caption}

Tumblr’s theme submission requirements:

Include {RSS} (never use “/rss”), {Favicon}, {CustomCSS}, {LinkOpenTag} (for Photo posts), {MetaDescription}, and {PhotoAlt}.