Hi Phillip
If you work off the computer screen a lot as I do and particularly if you use email a lot I use a screen grabber called Snagit…… Google will have no trouble finding it.
If I want to send or post or email an image at screen resolution in my case 1680X1050 pixels just open the image and size it to the size you want on the screen and the receiver will see a similarly (But not necessarily the same size image it depends on their screen resolution) sized image.
This size difference dependent on the hardware applies to any graphics file. Luckily the pixel resolution of most LCD screens these days is the over a fairly narrow range.
Of corse the recipient of the file you send you can scale the image up and down on their screen however the image is best seen at 100% (meaning as made resolution).
Even if file of the image is at extremely high resolution Snagit will grab it at screen resolution. It is a simple matter to paste it directly on an email or save it as a file (in twenty eight different graphics formats).
I am talking here of optimising the file size of images to be viewed on a computer. If the image is to be processed, or enlarged then it is better to use the full size file. although as you indicated large files are problematic to email.
To use Snagit you simply press the print screen button on your keyboard and a cursor pops up allowing you to select any window, the entire screen, or a fraction of the screen. You can also annotate the image with arrows (and a lot of other graphics), or text before saving or pasting it. It automatically saves a slide show of all the images you grabbed. I use this program several times a day
I have no connection with the company. But do highly recommend the program.
Cheers
John