FWIW. When an image is displayed on your screen, somewhere there is a little box telling you it's displayed at, say, 20% or 30% or whatever
Also somewhere on your computer you'll have an image management programme. If you right-click on the image file and select "open with", it will probably give you some options
A couple of suggestions:
Open the image in the image-management application of your choice
Somewhere in the "edit" area there will be an option "resize"
If the image is currently displaying at, say, 25% to fit on your screen - then resize it to 23% or 24% and you'll find it now displays on your screen at 100% or close-enough. Save it as a JPEG with another file name to distinguish this small image from the original
In terms of the number of bytes of digital storage the image occupies - you will probably find it's now 1/10th or 1/15th or some such of the original digital file size
I would think that if you upload it to the site using the "Attachments and other options" feature just below the dialogue box where you type your reply/message - it should load easily and open as a fits-on-a-page image
Just saying . . . Cheers