Some edits to "Using desktop IDEs" page and some notes about images

[isbur]

• Additional screenshot to "Using desktop IDEs" page in user-guide of where SSH button is (took me some time to realize, even though it is present on next screenshot on the page)
• While adding image I noticed that directory for images in readme.md isn't correct: one last subdirectory codenvy is missing
• And I struggled a bit with my screenshot resizing so I decided to add a note about it next to information about image linkage rules