JPEGs and PNGs can both be used, but JPEGs are the most common and highly recommended for most situations
PNGs have a transparent background
JPEGs have a solid background
Only use a PNG when you do not want a solid background on your image
Use JPEGs all the other times
File type can also depend on the purpose of the image
Usually banner images and content images are JPEGs
Icons depend on the use case but are most commonly either PNG or JPEGs.
Size (dimensions)
All image dimensions should be set to be only as big as the area in which they will be displayed
For example: if the content area where you will place your image is only 400x600px (pixels) then you should only upload an image that is 400x600px. Anymore pixels than that will just eat up space on your server and slow your page speed down.
Don't try to make a small image larger than it already is
Make sure your image file size is under 100 KB (kilobytes)
Resolution
Having a large scale, high resolution image is helpful to start with, but never upload a high resolution image
All imagery and media should only ever have a resolution of 72 PPI (pixels per inch).
The web only processes imagery at a resolution of 72. Anything more than that will unnecessarily eat up space on your server and slow your page speed down.
Quality
It’s possible to adjust the quality of your imagery, thus reducing the file size, without impacting the visible quality of the image.
Using tools like Photopea.com or Photoshop (or other free online tools) will allow you to adjust the quality of your media.
Don't try to make a small image larger than it already is
