If attaching here (so they appear in post as a clickable thumbnail, selected using the paperclip icon), it needs to be a jpg and less than 97kB - so check before uploading or you'll have to do it again. (everytime I do this, you'd think I'd learn

)
If I select the 640 x 480 size when reducing, I find the file is still bigger than 97kB, so you may need to go smaller still, and/or select a somewhat lower than maximum jpg quality).
The other method; to make a picture actually appear within the post itself, is to upload them tp somewhere else on the web and get a direct link (URL) to that photo location and use that in the post for an image address (using the little mountains icon), it is when you do it this way that the 700px max width is actually most appropriate.
In either case, there's a FAQ on this sort of thing, and also the
Suggestions & Feedback forum, so try there for beter guidance and the absolute rules rather than my version.
Hope this helps, Dave