Images are often referred to as hi-res (high resolution) and low-res (low resolution). A high resolution image intended for print must have be least 300dpi and a low resolution image which is intended for screen display (web) is around 72dpi.
DPI stands for Dots Per Inch.
Without getting too technical, imagine an area on your image (an inch in measurement) and when you look at that area the amount of dots in the image you are getting for low-res is only 72 dots, but if it was hi-res it would be 300 which means that there is much more detail in the hi-res image as there are more dots, more detail, more information.
OK, so you have your 300dpi image now, BUT wait a minute, how big is the actual file in size? If it is, say, 50mm in width, but you want to use it on your poster at 100mm this isn’t going to give you good quality, because you are then making the image 100% bigger than it is intended to be used at the 300dpi file size. So now it only becomes 150dpi and now not good enough quality to print from.
So not only do you need to have a 300dpi image, if you intend to use it large, it also needs to be a big size file in regards to measurements of width and height too.
There is no point giving me a 300dpi file, if it is really small and you want it on a massive banner. If it gets enlarged to over 200% you are making the quality poor once more.
How do you check if your image is 300dpi? If you have the right software for example photoshop, you can open the file up and check the image size – in there you can see the height, width and pixels/inch. When using this image, make sure you are not making it larger than the original and you will get the quality results.