Most web services that allow you to upload photos have limits on the size of the image you can upload.
These are the file size limits for the following services:
Gmail: 25 MB (each email can have 25 MB attached)
Moodle: 64 MB
Tumblr: 10 MB
Prezi: 50 MB / 2880 by 2880 pixels
Workable dimensions for most web content such as blogs hover around 640 x 480 pixels.
It is also important to resize images for use in PowerPoint, particularly when using multiple images. Using needlessly large images can make your PowerPoint file load very slowly. The ideal size for images in a PowerPoint is 1024 x 768 pixels, because this is the standard resolution that most monitors and projectors can handle. Larger images will make the file load more slowly without the picture looking any better, and anything smaller will start to pixelate when it is enlarged.
Read more step by step instructions for Mac and PC.