when you upload, on that page, it says do not enlarge your images, upload them as is. they do expect you to look at the image at a 100% on your pc before you send them here. the site never said anything about using 300dpi. there are no max or min sizes. you upload the size you have.
again, forget you know anything about dpi at all. upload the image at the size the image was born with. faa prints at 88-100dpi for the largest sizes. where as other companies have to use 300dpi to get a fine print, this site has a different method and can do it with less. however it comes at a cost, the image has to be pretty freaking awesome up close. and they won't print it if it looks bad.
don't downsize, the damage is already done. you'll have to go back to your original edits or edit them again. its like silly putty, if you grab snoopy from the comics and stretch him, he's now faded and distorted, and no matter how even you push it back down, snoopy will always be loopy.
you can ask that person what they meant by hi res - and they may be doing it wrong as well. the site won't tell your its wrong until it sells. and then you lose the sale if you can't fix it.
---Mike Savad
MikeSavad.com