A Vectored Image (such as; CDR, Ai, EPS etc.) is one where the individual & separate component pixels within the image can be manipulated ie, in movement, size or colour, independently. These are preferred/required for most printing jobs. Vector images can be printed as large as require without pixelation occuring.
A jpeg (a type of raster image such as jPEG, PNG or GIF) is a single, one piece image like a family photo not suitable for independent component manipulation or blowing up too much. We are all familiar with the pixelation that occurs when a jpeg image (family photo) is stretched too far.
Some photographs when taken at a high enough resolution (eg min 300dpi for a 120mm image) can be used for small images (or even large ones if they are to be viewed from a distance and pixelation isn't an issue), however printing from photographs, particularly those transmitted electronically is unreliable and can lead to hours of wasted time trying to correct colour imbalances etc..
When creating a document for printing or reproducing in a large size ( more than a couple of inches or so ) it must be created in a draw programme such as Corel or Adobe Illustrator and saved as a vector file not in a photo shop style programme (the clue is in the title-its designed to mess around with photos) which can only create jPEGs.
The temptation to add newly discovered tricks such as 3d, varied tints, shadows, etc., can also complicate issues.
When exporting files containing lines, outlines and fonts always convert the whole file to curves before sending to avoid scaling and font issues.
So, to summarise, the advice is:
- Always create vectors (& remember exporting a jpeg as a vector image does not make it a vector).
- Keep it simple and avoid being tempted into too many tricks.
- Send the file as curves not text to ensure compatibility.
Hope this helps.