Usually many personal computers or laptops.
May have programs already.
Click image .. Bringing it up for display.
Look and see if maybe top of display. Your offered resize or select in toolbar.
Select you usually can use, to maybe trim some of it.
Cutting out extra back ground on image, around sides.
Just make sure it is what you want before saving it again.
Also you can maybe rename it before saving. To keep original, and have posting copy.
One last thing .. Check and see what kind of Image it is.
Png Jpg .. so on.
I have found some times, when I download an image. That is a png.
It usually is huge file. Maybe a few MB in size.
If opened, and going to save it again.
Choose to make jpg copy/ convert it.
Size will drop down usually more than half file size.
1.5 MB image. May drop down to only 230KB or so.
Before trying to post it.
Hope something there helps.