Leanpub fixed new images

We found the following problems with your images:

- The image images/installation/ubuntu/firefox.png has a really low
resolution. We have fixed it for you. You don't need to take any
action. This image is used in these files: installation/ubuntu.md

- The image images/installation/ubuntu/firefox.png is too wide to fit
on your page. We have made it a bit narrower for you. You don't need to
take any action. This image is used in these file:
installation/ubuntu.md

- The image images/installation/ubuntu/chrome.png has a really low
resolution. We have fixed it for you. You don't need to take any
action. This image is used in these files: installation/ubuntu.md and
installation/ubuntu.md

- The image images/installation/ubuntu/chrome.png is too wide to fit on
your page. We have made it a bit narrower for you. You don't need to
take any action. This image is used in these file:
installation/ubuntu.md and installation/ubuntu.md

- The image images/installation/ubuntu/opera.png has a really low
resolution. We have fixed it for you. You don't need to take any
action. This image is used in these files: installation/ubuntu.md

- The image images/installation/ubuntu/opera.png is too wide to fit on
your page. We have made it a bit narrower for you. You don't need to
take any action. This image is used in these file:
installation/ubuntu.md