Hmm......according to Edit Profile (in my account):
Maximum upload file size: 2048KB.
Allowed Files: jpg jpeg png gif
Maybe the file size and the number of pixels people have suggested amount to the same thing? I agree any picture editor should be able to downsample a picture to the required size. Even Microsoft's "Paint" looks able to do that - if you can find Properties in the menu you can set the pixel width and height from there. Microsoft Office Picture Manager looks like it could also do the job, using Picture > Resize. Don't know if the modern versions of those programs have the resizing feature in the same part of the menu though.
Posting pictures on a thread is something I've never done. It looks like it should just be a matter of clicking Insert Image (the rightmost icon) from the icons at the top of the message box, then inserting a valid URL for the picture you want. I do know that it won't work if you just enter the path to an image file on your computer, so I suppose if your picture wasn't already on a website somewhere, you'd have to upload it to one, for example:
https://postimages.org/
Personally I'd prefer it if that wasn't necessary and WP would accept local images, but I suppose they might use up a lot of WP's storage space, so maybe it's better as it is.