We are happy to print from JPGs, we don't even need them to be CMYK. We convert the colour for you and check it looks OK. We add bleed by resizing slightly or we can add a border depending on your artwork, and we would supply a PDF proof back for you to approve first. It is better if your JPG is at least 300dpi but not essential. Due to the sophisticated equipment these days, I am often amazed by the results we get for jpegs with lower resolutions. I hope this helps
300dpi jpg in CMYK colour is fine for print. If the image is a graphic image and in RGB, stay well away from the printer converting the colours, some colours can turn to sludge, a real shocker if you are looking for perfection.
A 300dpi jpeg can be fine for printing, even a 150dpi one surprises me with it's print quality sometimes.
If your printer converts the colour and gives you sludge back you are going to the wrong printer! We convert colours but always supply a PDF proof, or warn the customer if there are going to be any colour shift issues. Also remember, a 300dpi jpeg can also be poor quality, depends on how good it was to start with and the level of compression applied.