If you are making a post here using photos, while writing a walk report for example, then you can do this in two ways:
You can upload the photo using the "Upload attachment" option, add the file, and then use the "Place inline" button to put the image where you want in the post (it appears using an [attachment] tag in your post at the point where you had the cursor before clicking the button).
You can use a photo you have hosted elsewhere on the Web, on Flickr say or Photobucket or even on your own web site and use an [img] tag to insert the URL of the photo into your post.
If you use the first method then our software will automagically re-size the photo to have a maximum width of 690px (so it fits the width of your post) and add a border.
Previously if you used the second method the photo would get chopped off to the right if it was more than 700px wide. We've now fixed this (via CSS) so that your photo is re-sized to be 690px wide. This should work on all browsers (although it's possible it won't work on old versions of IE ... but then what does ).
However be warned that if you use large images the viewer is still going to have to pull that big image down off site where the image is hosted as the re-sizing is done on the viewer's browser.
This is bad news in three regards:
Viewers on slow connections (2G mobile for example, lots of that about still) will not be happy with how long your walk report takes to load, especially if you've used a lot of images.
Viewers paying for the bandwidth (so mobile users mainly) won't appreciate your walk report chewing up their allowance.
If you value the quality of your images then in my experience web browsers make a pretty poor job at re-sizing photos on the fly, certainly far worse than Photoshop/GIMP et al.
So all in all it's probably in your interest to use an image size close to a width of 690px if you can when using [img] tags.