Costco Printing question

I photographed an artists paintings for her website and then she decided she wants a couple of the images made into posters. She went to Costco and asked for a 20x30" print of each and they told her there wasn't enough resolution.

I use a MKIII 10MP, gave her sRGB JPEGS, Exported out of LR3 at 100% quality and 300ppi (which Costco agreed was the ppi that they wanted) but the images had to be cropped to fit the narrow side so they aren't full size images, can't be. I checked with MPIX and they conferred that 20x24 was as large as the test ones I checked on can go. That all makes sense to me. But...

I have no dealings with Costco, a lot of photographers around her, yeah actually a lot of people who are selling prints are having them do their processing, I was supprised, it wasn't something I expected.

Do they, would they, could it help and can they print from a TIFF, is that done? Does it help?

Are sometimes posters made larger that recommended size (I think I've seen that before judging from the crummy quality)?

All of the printing I've had done before which is rare, has either been with Shutterfly, MPIX or my own HP B9180 which is A3 13x19" so I have no reference for this.

Resolution comes from the number of pixels in the image and nothing else so what size are the files? 2000x3000 pixels would allow printing 20x30 at 100ppi which is borderline for quality. If the files are larger than that, then they should be able to print the images easily enough. I've got poster prints from my Costco that were 140ppi and they looked okay.

BTW, .tif would not make any difference.

I use Costco for my printing and love it. What I do, however is use their web interface at http://www.costcophoto​center.com/ to upload my photos, choose my sizes and finishes, and most importantly turn off their "AutoCorrect" feature. I can then have the files sent to the store to be printed without them being altered at all. Using the web interface you can have any file printed at any size. Just today in fact I had a 1050x1226 JPEG printed on a 20x30" poster and to my surprise it came out pretty decent, considering it's going to be viewed from across the room. This is a link to their guidelines on minimum resolution for each print size. If my poster came out ok, I'm sure your 10MP file will look just fine. If she doesn't want to get set up on the web site, just have her tell them to go ahead and print it.

Another tip: Costco lets you download custom printer profiles here for each of their stores so when you export JEPGs to send to them, the printed results are perfectly color matched

Jannie wrote in post #11908496
20x30" print [...] the images had to be cropped to fit the narrow side so they aren't full size images, can't be. [...] 20x24"

20x30 and 20x24 are different aspect ratios.

A 10Mp image should make a decent 20x30" image

High quality jpg or tiff is about identical for printing.

Native 1DIII resolution printed to 20" height will net 130 pixels per inch on print. While other responses (post 3 by ATCG) have indicated Costco does take small files and print large, you can always resize the file 230% in both directions, to yield 8972x5981 pixel image which allows the 300 ppi density which you were told you 'have to have'