Don't use for...
|JPG||Raster; lossy compression (small file sizes but loses quality)||Photos and artwork- online and printed||Transparent web graphics|
Raster; lossless compression; allows transparent background
|High quality transparent web graphics, such as logos, charts and icons||Photos|
Raster; lossy compression, small files, limited colors
|Animated web graphics||Printed graphics|
Raster; very large files, uncompressed, detailed image data
|High quality print graphics, high quality scans||Web graphics|
|Vector; preserves formatting||Printing, displaying web documents||Editing designs|
More information: Image file formats: when to use each type (99designs blog).
Raster images (.jpg, .gif, .png, .tif.) are comprised of pixels. If you enlarge a raster image too much, it will be blurry and pixelated.
Vector images (.ai, .eps, .pdf, .svg) are comprised of paths. They stay sharp and crisp regardless of how they are resized.
Lossy compression creates small file sizes, but loses quality at higher compression ratios.
Lossless compression does not create a loss of quality, but file sizes are larger than lossy compression.
Image size refers to the dimensions of the image in pixels. For example, a 3x5 inch image translates to 300 x 500 pixels.
Image resolution is measured in dpi (dots per inch) or ppi (pixels per inch).
Screens have a resolution of 72ppi, so if you are creating digital graphics, you don't need a higher resolution than 72ppi.
High quality printing prints at 300dpi.
If you need to change the size of your image, make sure to keep the dimensions proportional. Squishing or stretching your images can make your work look less polished.
It depends on the software you are using, but you can lock the proportions so that you don't warp your images when changing the size. Often holding the shift key while changing the size will lock the proportions.