How do I make a normal JPEG image into a poster print?

Answer 1

Problems you will face here:

1. The PPI of the JPEG
2. The original size of the JPEG
3. The color properties will be wrong

If you have a JPEG that is already the size of your poster you want to print, you can pull that JPEG into Adobe Photoshop to change the color space. You will want to take the JPEG from RGB to CMYK. It will not be perfect but it will be better than just printing something in RGB.

Next if you JPEG is smaller than you poster size, you have a real problem. The JPEG is a raster image therefore it will not be able to stretch to larger sizes without getting pixelated.