I am fairly new to adobe and all things it has to offer. I am slowly learning how to do everything myself. I am currently using CS6. I made myself a 16x20" poster in Illustrator, converted it to a .jpg using the export menu. I put it to my flash drive and printed it out at a local fed ex office. When converting it to jpg, I kept it in CMYK color mode, and set it to be at the highest resolution and the highest quality file I could have.
Now, I made another poster for a friend and did the exact same thing. Instead of taking her .jpg somewhere to get printed, she is trying to upload it to a photo printing site (walmart, snapfish, shutterfly) to get it printed and mailed to her. Her poster keeps getting errors on the photo uploading site saying it is corrupted and not a valid file type. I was confused, so I tried messing around with the illustrator file a bit, I ended up changing the color mode to RGB when I converted it to a jpg (I still made the artwork in CMYK mode). This uploaded fine to all the photo sharing sites, and was also half the size of the original .jpg (12 MB vs. 24). So then I googled it, and came across a website that says printers only print in CMYK. So if that is the case, what will happen when she prints this poster? And why won't these photo uploading sites allow me to upload a poster that is in CMYK mode? If printers only print in CMYK, you would think it wouldn't be a problem.
On a side note, when I try to upload the CMYK .jpg image to Fed Ex office online, it perfectly uploads. So maybe this is just a problem with simple photo uploading sites?
Thanks for the help! Again, I am fairly new to illustrator so I don't understand color modes much or much else.
The first rule of getting something printed is to know what the printer (in one of the senses machine, company, or (wo)man) needs/wants. Hopefully (sometimes only hopefully) that is what results in the best possible print; otherwise you may consider using another printer (in any of the senses).
Most printers use CMYK and maybe some other colours, but only some printers, such as offset printers and PostScript (driver) printers apply CMYK directly; others apply RGB and convert.
Obviously, there is much more to be said about the subject.
And why won't these photo uploading sites allow me to upload a poster that is in CMYK mode?
Since they do automated conversions to their device profiles based on their own profiles - just liek any home printer does. They all assume a standard sRGB color space, since naturalyl 90% of al lpeople never have heard anything about color management, color profiles and so on and simply upload their photos directly from their digital camera. In fact I would consider any print booth that uses a CMYK file without warning more dangerous... anyway, as Jacob said, you have a lot of reading to do. that's nothing someoen can explain in 5 simpel sentences.
I suggest you get your hands on Adobe's Print Publishing Guide. I am sure there are many online "photo" print services out there. The key is they are "photographic" printers which are RGB based. It wouldn't surprise me if they incorprated some type of default rejection coordinance where they reject CMYK files. You should be looking at PDF instead of JPEG anyway. And, since that is a large format print job, a medium resolution based PDF is adequate and small enough to send FTP. The advantage to PDF is it can honor a well established color workflow. Focus on reading the various RGB color spaces and the different levels of PDF. Also, spend some time on color management and converting from RGB to CMYK. I'd be interested in learning which color settings and workspace profiles you have in place.