Butterick’s Practical Guidance is clear, "A first-line indent on the first paragraph of any text is optional, because it’s obvious where the paragraph starts.”
I understand this applies after a section heading.
Does it also apply after a page width graphic?
Does it apply after block quotations; and if indent is appropriate after a block quote, should the indents match?
No doubt these are rookie questions and any thoughts you can offer would be appreciated.
Like all advice in the book it is subject to the judgment of the person doing the typesetting. As a reader, too many exceptions to the first-line indent could possibly create ambiguities. For instance, if you omit the first-line indent after a blockquote, then a new paragraph and a continuation of the same paragraph would look the same.
I decide whether to indent and the size of the indent based on the reading material. I follow Chicago’s rule for quotations, which I’ve included below this post.
I think it’s important to be visually consistent with text, so if the entire work has indents, then I would suggest that any text below a graphic should also be indented.
Another factor I consider is the rhythm, tone, and the voice of the writer. Content without indents has more reading tension and requires a clear transition at the end of each paragraph or the reading material can get choppy. Making the next paragraph into a read bump of sorts. Material without indents should also have a strong narrative that causes the reader to continue reading without hesitation.
I’ve become interested in reading tension over the past few years, which causes me to advise a writer to use indents unless they’re writing a business letter, email, the text is in columns, or on a webpage and/or post such as this one.
Quoted material of more than a paragraph, even if very brief, is best set off as a block quotation. (For a less desirable alternative, see 13.32.) A multiparagraph block quotation should generally reflect the paragraph breaks of the original. But if the first paragraph quoted includes the beginning of that paragraph, it need not start with a first-line paragraph indent. Subsequent paragraphs in the quotation should be indicated either by first-line paragraph indents or (less desirably) by extra line space between the paragraphs (see also 13.24).
If the first part of the opening paragraph were to be omitted, it would still begin flush left. For ellipses at the beginning of paragraphs, see [13.56]