Advice, please: dpi for poster images?

Are there recommended guidelines or rules of thumb that I should follow?

Yes. Since before the beginning of PostScript, the rule-of-thumb for commerical offset lithography for digital continuous-tone images is ONE-TO-TWO TIMES THE HALFTONE SCREEN RULING. It is NOT the widspread misconception of a blanket "300 ppi for anything in print."

Yes, there are esoteric details which some will trot out to unnecssarily obfuscate the issue. But if you understand the basic purpose of the rule-of-thumb, its principle becomes common-sense: The purpose of a minimum ppi is to eliminate visible "pixelation," often called "the jaggies." The operative word here is visible.

Now try this mental exercise: You have one round object (a halftone dot). Use it to render for me a square.

That's right; you can't. If the number of pixels in the image is at least equal to the number of halftone dots with which it will be rendered, there is no way you are going to see the square shapes of individual pixels. The rule-of-thumb adds some "leeway" to allow for on-page scaling (which is not best-practice anyway).

Over the years, too many people, who either forgot about or never understood the basic principle in the first place, corrupted the rule-of-thumb to the unthinking, too-often bantered "everything has to be 300 ppi" misconception. People go around saying this without even mentioning the factor required for a number: the halftone ruling! That's a dead givaway that they don't really understand the principle they go around spouting their "expert" advice about. They get this "300 ppi minimum" from the assumption that the halftone ruling is the common magazine-quality ruling of 150 lines per inch (an assumption you should never make). They then think, "well, if the rule is 150-to-300, best quality must be 300."

That's like reading the maximum inflation pressure marked on your tires and calling it optimum.

It's like Microsoft assuming that if two mouse buttons are good, then 12 must be great!

Look at the rule-of-thumb again. It is a RANGE. It's a range from acceptable on one end (undersampled) to acceptable on the other end (oversampled). What does common sense tell you is optimum within that range?

I typically try to stick with a minimum resolution of 200  (preferably 300 or more) dpi for print items.

You should now understand my asking this question: Upon what criteria have you been basing your own rule-of-thumb? By what principle and in what circumstance do you consider "300 ppi or more" preferable?

I'm working on some 18x24 posters and will be doing some even larger-format display pieces in the coming weeks.

It should now be clear that asking a question about raster resolution is nonsense unless you provide some basic information about the printing method for which the documents are being prepared. What is the repro method? (Litho offset? One-off inkjet printing?) What will be the screening method? (Traditional halftone? Stochastic screening?) What about the images themselves? (Line art? Continuous tone?)

Trying to provide "expert" answers without raising those questions is just as nonsensical, and throws doubt upon the credibilty of the "expert."

JET

An 18x24 is probably going to be able to be printed on an offset press.  It could also be printed on an inkjet, large-format printer.  As far as images are concerned, their resolution depends on which print method you are planniing for.  If you're planning for offset press, 300ppi at 100% final size is a good choice.  If you're planning for inkjet, 144ppi at 100% final size is acceptable.  Getting photos that big is a challenge.  I use Genuine Fractals plug-in in Photoshop to size my photos.  There are other methods, too.  Most stock-photo agencies supply photographs in multiple resolutions and sizes.  If you are working with digital camera images, use the camera's largest capture size and then size the photo(s) accordingly.

It depends on the quality of the final printed piece you require.

High quality is 355 for many printers though 300 should be excellent.

Large format can be had for much less and mural size can go down to 72 ppi and some pre press folk here have gone down eve further.

You really discuss it with the printer.