Resolution
All files and images must have a minimum of 300 dpi resolution to print sharp, high-quality images. Images on the web are often created at a low resolution of 72 dpi — ideal for websites to load faster, but NOT ideal for print.
DPI = Dots Per Inch. A higher DPI means higher image resolution, better quality, and larger file sizes.

72 dpi — Prints fuzzy / blurry
Good for web, not for print

300 dpi — Prints clear
Good for print
Tip: Zoom in to 300% on your screen to see how your image will look at print resolution.
Can you change my low-resolution picture into a high-resolution picture?
We CAN make a high-resolution picture into a low-resolution picture, but we CANNOT make a low-resolution picture into a high-resolution picture. If your file is lower than 300 dpi, you can make the picture smaller (which increases the dpi), or buy, find, or take a new picture at a higher resolution. Stretching a low-resolution file to a bigger size makes it even lower resolution and will print blurry or fuzzy.
This concept is similar to tailoring clothes. You can cut and hem pants to make them shorter, but you cannot make them longer than they already are — there simply isn't enough fabric.
How do I get a high-resolution picture?
3 ways to get high-resolution pictures:
Take a picture on your own camera or phone camera

Purchase a high-resolution stock image from an online photo gallery
Search for a high-resolution picture on the web — note that most web images are low-res (72 dpi).
If you save a picture from the web, make sure it is large enough. Do not use copyrighted images.
Go to images.google.com and search for what you need.

On the results page, click "Tools" then the "Size" dropdown to filter by image size.

Choose "Medium," "Large," or "Larger than..." — the bigger the image needs to print, the larger the source file should be.
Minimum pixel size = (print width × 300) × (print height × 300)
Example: A 4"×6" postcard needs an image of at least 1,200 × 1,800 pixels.