there are a number of ways to do it...I take pics with my phone and email them to myself, open them on my computer and use the 'snip' feature (known as the Snipping Tool on windows machines) to copy the pic and save it to my 'scooter forum' file. The 'snipped' pic will be a much smaller file size than the original.
The snipping tool is a great thing to use, it allows you to copy full or only parts of a screen, save, and send to someone. I use it for work all the time. Do a search for it on your computer, open it and then pin it to your task bar for easy access.
Others will upload pics to a 3rd party site (Flickr, etc.) where you can change the size of the file. My previous email program let me alter the size of a file I was sending in email, so I could choose a lower resolution / smaller file size.